How do you measure a spaceship that does not currently exist? For example, what is the size of the Klingon Bird of Prey compared to the Starship Enterprise NCC-1701-D?
Or maybe you want to know the scale of the Martian Tripod From War of the Worlds compared to Centauri Primus from Babylon 5?
Starship Dimensions sets out to help television and movie science fiction fans view a meter to pixel rendition of their favorite ships. You can also compare ships to current real life objects.
New ships are being added all of the time but there is enough to keep a team from Memory Alpha busy for a week - http://www.merzo.net