I have been away from the hobby for a long time, but have gotten back into 3d modeling after picking up a resin printer. I mainly create miniature starships from different science fiction settings, including Babylon 5 and Star Control, along with custom ships of my own design.
My ships are designed to fit in my own "Trata Kam" Scale, which I have found makes ships large enough to be interesting to paint but not so large as to be too difficult to use on the table top:
Fighters: 15mm
Corvettes: 20mm
Frigates: 30mm
Destroyers: 40mm
Light Cruisers: 50mm
Heavy Cruisers: 60mm
Battlecruisers: 70mm
Battleships: 80mm
Dreadnoughts: 90mm+
The end result is that these ships can generally be scaled up without much problem, but scaling them down may be more problematic.