100 studs is a lot of ship (31.5 inches or 80cm for those counting). I haven't built anything that big since before the Dark Ages. My latest vessle, the HMS Desmond Hume, is only 24 inches.
Compare that to the USS John Bell Hood, at 28in, and you can tell that I'm really cheating with these huge booms on the front.
I consider the Hood to be more of a Battlecruiser due to it's heavy weapons, and the Hume a sort of missile frigate, primarily because I forgot to put weapons on it until the very end and decided to put those recessed tubes on the sides.
I think a better indicator of SHIP size would be Displacement, rather than Length. The USS Robert E Lee, while only 22" long (about 2/3rds of S.H.I.P. status), is nearly 800 cubic inches, over twice that of the John Bell Hood and about three times the Desmond Hume's volume:
Basically I think ship classification should be by interior volume, rather than just length, which seems kind of arbitrary and sometimes leads to people like me putting huge antennae on the front just to get that extra few inches.