This is taken from a post by Iuris, reposted by Horizon as well.
BFG Sizes Warseer
Here are the sizes from the Rogue Trader books...
IMPERIAL NAVY and CIVILIAN VESSELS
Avenger - 7.5 km x 1.8 km, 40 mTon, 141.000 crew
Repulsive - 7.4 km x 1.5 km, 39 mTon, 134.000 crew
Exorcist - - 7.3 km x 1.3 km, 37 mTon, 112.000 crew
Overlord - 5.3 km x 0.85 km, 31 mTon, 100.000 crew
Chalice - 5.1 km x 0.8 km, 29 mTon, 98.200 crew
Armageddon - 5 km x 0.8 km, 30 mTon, 98.500crew
Lunar - 5 km x 0.8 km, 28 mTon, 95.000 crew
Conquest Galleon - 5.1 km x 0.7 km, 30 mTon, 65.000 crew (note: half-transport)
Ambition - 4.9 km x 2 km, 25 mTon, 89.000 crew
Dictator - 5.1 km x 0.8 km, 29 mTon, 85.000 crew (carrier, +15.000 pilots/support personnel)
Dauntless - 4.5 km x 0.5 km, 20 mTon, 65.000 crew
Endeavour - 3.8 km x 0.5 km, 22 mTon, 67.500 crew
Defiant - 3.8 km x 0.5 km, 22 mTon, 61.500 crew (carrier, +6.100 pilots/support personnel)
Sword - 1.6 km x 0.3 km, 6 mTon, 26.000 crew
Tempest - 1.5 km x 0.4 km, 6.1 mTon, 30.500 crew
Falchion - 2.2 km x 0.3 km, 6.5 mTon, 28.000 crew
Claymore - 1.4 km x 0.3 km, 5.9 mTon, 21.000 crew
Turbulent - 1.95 km x 0.3 km, 7.5 mTon, 25.000 crew
Hazeroth - 1.5 km x 0.25 km, 5 mTon, 22.000 crew
Havoc - 1.6 km x 0.4 km, 6 mTon, 24.000 crew
Meritech Shrike - 2 km x 0.25 km, 4.5 mTon, 15.000 crew (Heretech vessel)
Iconoclast - 1.3 km x 0.4 km, 6.1 mTon, 16.500 crew (Destroyer)
Viper - 0.95 km x 0.25 km, 4.9 mTon, 7.500 crew
Jericho - 2.25 km x 0.3 km, 9 mTon, 20.000 crew
Vagabond - 2 km x 0.4 km, 8 mTon, 18.000 crew
Carrack - 2.1 km x 0.4 km, 8.5 mTon, 19.500 crew (combat adapted)
Goliath - 4.9 km x 0.9 km, 16 mTon, 41.000 crew (onboard plasma refinery)
Universe-class Mass Conveyor - 12 km x 1.3 km, 60 mTon, 60.000 crew (+200.000 to 500.000 passenger capacity)
(data will vary due to individualistic nature of the mekboyz)
Hammer Battlekroozer: 4.8 km x 1.2 km, 30 mTon
Kroozer: 4.5 km x 1.1 km, 30 mTon
Attack ship: 1.5 km x 0.4 km, 9.5 mTon
Brute ram ship: 1.1 km x 0.5 km, 6.5 mTon
Rok: repurposed asteroid of up to 4 km
Shadow cruiser: 4.7 x 1.4 km, 16 mTon
Eclipse cruiser: 4.7 x 1.4 km, 18 mTon
Aurora light cruiser: 3.4 x 0.4 km, 12 mTon
Hellebore frigate: 1.8 x 0.3 km, 5 mTon
Hemlock destroyer: 1.4 x 0.2 km, 4 mTon
Nightshade destroyer: 1.3 x 0.3 km, 4 mTon
Shadowhunter escort: 0.8 x .015 km, 3 mTon
Wraithship cruiser: 3 x 0.4 km, 12 mTon
Dragonship cruiser: 4.5-5.5 km x 0.4-0.6 km, 16-20 mTon
Butcher transport: 2 x 0.6 km, 9.4 mTon
Mauler frigate: 1.3x 1.0 km, 8.4 mTon
Mangler light cruiser: 3.1 km x 1.2 km, 18 mTon
Warsphere battleship: spherical, 9km diameter, 47 mTon, 300.000+ kroot
Soulcage slaveship: 2km x 0.5km, 10 mTon, 30.000 crew + thousands of slaves (captured slave transport)
Hellbringer planetary assault ship (light cruiser): 4km x 0.5 km, 22mTon, 40.000 crew + 100.000 raiders
Devastation cruiser: 5km x 0.8 km, 28.5 mTon, 80.000 crew
Retaliator grand cruiser: 6.2km x 1.4 km, 38 mTon, 140.000 crew
Pestilaan light cruiser:5km x 0.9 km, 39 mTon, 50.000 crew plus "countless lesser demons"
Slaughter class cruiser: 5km x 0.8 km, 28.5 mTon, 80.000 crew
Carnage class cruiser: 5km x 0.8 km, 28.5 mTon, 80.000 crew
Hades class heavy cruiser: 5.2 km x 0.8km, 33.5 mTon, 130.000 crew
Infidel class raider: 1.5km x 0.25 km, 6 mTon, 24.000 crew
SPACE MARINE VESSELS
The Hunter-class destroyer, although based on a design long available to almost all Space Marine chapters, was pioneered primarily by the Dark Angels, whose deep distrust of other factions within the Imperial hierarchy made acquisition of vessels from shipyards and forgeworlds far more problematic than for most other chapters. As such, the Dark Angels chose the Hunter-class destroyer as the most viable counterpart to the Imperial Navy’s Cobra and maintains large flotillas of the vessel in all their battlefleets.
Hull Type: Raider
Dimensions: 1.3km long, 0.3km abeam at fins approx.
Mass: 5.2 megatonnes, approx.
Crew: 18,500 crew, approx.
Accel: 6.7 gravities max sustainable acceleration
Hull Integrity: 35
Turret Rating: 1
Weapon Capacity: 1 prow, 1 dorsal
Of all the vessels in the service of the Adeptus Astartes, the Nova-class frigate is commonly the single class to which the Imperial navy takes the most exception. It lacks sufficient size to really fulfill the deployment and assault roles for which Space Marine ships are primarily intended and its lance armament an speed make it a menacing gunboat in its own right. As such, the Nova remains rare in most Space Marine fleets, a trend the Imperial Navy, the Inquisition, and other institutions perpetually concerned by the balance of power would dearly love to see continue.
Hull Type: Frigate
Dimensions: 1.5km long, 0.62km abeam at fins approx.
Mass: 5.7 megatonnes, approx.
Crew: 20,000 crew, approx
Accel: 6.1 gravities max acceleration
Hull Integrity: 42
Turret Rating: 1
Weapon Capacity: 1 prow, 1 dorsal
Invariably the Gladius is the most numerous escort within a chapter-fleet, being one of the few escorts in relatively uniform usage across the chapters. The vessel, although larger than any genuine gunboats likely to be in use, is relatively small by Space Marine standards, seldom carrying mush more that a single squad of Space Marines who are invariably spread across its decks and control centers supervising the much more numerous chapter serfs upon whom the chapter relies for actual operation of its vessels. As such, the Gladius lacks the genuine punch of other Space Marine vessels, and is of little use in their favored planetary assaults, but instead earns itself an admirable place as the foremost ship of the line when it comes to more mundane border patrols and deep space blockades.
Hull Type: Frigate
Dimensions: 1.8km long, 0.7km abeam at fins approx.
Mass: 5.8 megatonns, approx.
Crew: 23,000 crew, approx.
Accel: 5.4 gravities max acceleration
Hull Integrity: 45
Turret Rating: 2
Weapon Capacity: 2 dorsal
While a Space Marine Chapter only rarely employs the might of its battle barges, Adeptus Astartes strike cruisers are a more common, although still rare, sight. Often the arrival of a Space Marine strike cruiser is enough to quell a rebellious system. The Space Marines are quick to act if their enemies’ surrender is not immediately forthcoming.
Strike cruisers are fast, lightly-armed vessels which mass slightly less than the Imperial Navy’s Dauntless-class light cruisers. Their primary function seems to be that of rapid response, reports indicating that they are invariably the first craft to arrive at a threatened planet.
Strike cruisers appear to carry approximately one full company of space Marines (including support vehicles) and have been observed to deploy them with in twenty minutes of arrival in orbit.
Like Battlebarges, strike cruisers do not represent a single class of vessel, or specific configurations of weapons and systems, but rather represent a broad range of different Space Marine vessels used for largely similar tasks. Strike cruisers, first and foremost need to be swift vessels, with a substantial transport capacity and various means of delivering those troops (be that a mixture of teleporters, drop pods, or launch bays equipped with Thunderhawks). Within these parameters, strike cruisers can, and do, take a variety of forms.
The Grey Knights, perhaps the most secretive and unusual chapter of all, have their base within the Solar System itself, within easy striking distance of Mars and hence their strike cruises are sleek vessels, utilizing quite simply the most advanced systems available, even by Space Marine standards. Other chapters, too, possess widely different forms of strike cruiser, each shaped by centuries of tradition and varying doctrine amongst the chapters of the Adeptus Astartes.
Space Marine Strike Cruiser
Hull Type: Light Cruiser
Dimensions: 4km long, 0.9km Abeam at fins approx.
Mass: 19.1 megatonnes, approx.
Crew: 60,000 crew, approx.
Accel: 4.9 gravities max sustainable acceleration
Hull Integrity: 60
Turret Rating: 2
Weapon Capacity: 1 dorsal, 1 port, 1 starboard
Most Space Marine Chapters control two or three battle barges. They are very brutal vessels, with only one purpose behind their design. As might be expected, a battle barge, is configured for close support of planetary landings and carries numerous bombardment turret and torpedo tubes. A considerable amount of hull space is given over to launch bays for intra-system craft and drop pods, observations indicating that up to three companies can deploy simultaneously. The vessel is extremely heavily armored and well shielded, presumably so that it can breach planetary defenses without harm coming to its cargo. Naturally the battle barge would make a frightening opponent in any situation where boarding is involved.
Space Marine Battlebarge
Hull Type: Battleship
Dimensions: 8.2km long, 2.3km abeam at fins approx.
Mass: 52 megatonnes, approx.
Crew: 130,000 crew, approx.
Accel: 2.5 gravities max acceleration
Hull Integrity: 120
Turret Rating: 3
Weapons Capacity: 1 prow, 2 dorsal. 2 port, 2 starboard