A useful site giving visual representations of the size of many SF vehicles.
Useful for me since my "fighter shuttle" size in my 4X game turns out to be a nice match for a Boeing 747 - just the size of the Orion Shuttle. Which means, yes, most SF starfighters are G-class units (the equivalent of a grav tank or atmospheric vehicle). [H-class is human by the way. The largest class of "ship" is the A-class Assault vessels (a Dreadnaught by any other name would still cure constipation in an enemy), although star bases and battlestations can go to Z-class and above. Unfortunately I don't quite make P-class = planet, although S-class may make a star...]