I tend to base it mostly on hull size, as follows:
Light things that I don't use much; Corvettes, Lancers, &c: 1-3
Destroyers / frigates: 4-5
Light cruisers: 6-7
Cruisers: 8-9
Heavy and battlecruisers: 9-11 (there's some overlap / discretionary here)
Battleships: 11-14ish
Dreadnoughts: >14
I also like to use MoO3's naming convention by hyperdrive capability / mobility, size, and task / purpose, so a jump capable point-defense destroyer mark n is denoted StrDesPtD-n (STaRship, DEStroyer, PoinT-Defense), while a battleship-size orbital fighter base would be OrbBatCar-n (Orbital, Battleship, Carrier, mark n). The function classes, I think, were long-range attack (LR), short-range attack (SR), recon / picket / EW (Rec), carrier (Car), point-defense (PtD), colony (Col), outpost (Out), and maybe a few others I'm forgetting, while the mobility classes were starship (Str), system ship / monitor (Sys), and orbital (Orb). It's a fun naming scheme.