Size comparison image of ships from all the following genres:

Star Wars (Expanded universe, G-Canon whereever possible)
Star Trek (official stats)
Stargate (official stats)
EVE Online
Warhammer 40.000
Babylon 5
Robotech/Macross
Halo
Battlestar Galactica (both series)
Firefly/Serenity
Farscape
Titan A.E.
Legend of Galactic Heroes
Starship Troopers (TV and Movie)
Aliens vs. Predator universe
Lexx – The Dark Zone
Red Dwarf
Event Horizon
V
The Irresponsible Captain Tyler
Martial Successor Nadesico
Space Pirate Captain Haarlock
Silent Running
Independence Day
Wall-E (Axiom, Zephyrus and Epiglottus city ships, based on the size calculation here: buy-n-large.livejournal.com/17″¦ )
Dead Space (sizes mostly based on Dead Space Wikia and conjecture)
Star Citizen (Size based on the Star Citizen Wiki)
Galaxy Quest (Sizes taken from a Chart compiled by Dan Carlson, who inspired me for this chart)
Space: Above and Beyond (Sizes taken from a Chart compiled by Dan Carlson, who inspired me for this chart)
BattleTech (several Inner Sphere and Clan ships; based on www.sarna.net )
Wing Commander (Sizes mostly via wcnews.com )
StarCraft (Sizes and ship images taken from: i2.minus.com/idKJOHIciRf0E.jpg )
Homeworld (based on conjecture, the Homeworld wiki which has everything except sizes, and a few fanmade charts)
Close Encounter (image via Smithonian; size conjecture based on film clips)
District 9 (Image internet; size wikipedia)
Romulan Mining Ship, USS Odyssey, and USS Vengeance (Dreadnaught-class) for Star Trek