Yes that's true, frigates are "usually" smaller than destroyers, and "usually" bigger than corvettes, if they aren't in general
For modern warfare terms, a frigate is something that's smaller than a destroyer and bigger than a corvette. It doesn't matter what an 1800's ship was considered to be a frigate or not. So what's like everyone going on about? You know there are ship classes bigger than destroyers that aren't in fs2. Like carriers and stuff. There is juggernauts, maybe they should be renamed as carriers or something, the sathanas releases a lot of fighters, and doesn't have much weaponry (thinking out loud). Then again, the problem with destroyers in fs is that they are given a carrier and a destroyer role, but you would expect space ships to function and have different habits and roles anyway, especially in size
In fs, ship sizes seem to be obvious and go up, cruiser, corvette, frigate, destroyer. The ships in fs2 are different than our modern warfare counterparts, because in fs2 they serve different and mixed roles. As far as modern day warfare and fs2 have in common seem to be the usual sizing category in tact with mixed role, like cruiser...corvette...frigate...destroyer.