Finally found my current (Unity) guidelines I'd made a while ago:
Escorts (Corvette, Frigate, Destroyer) 2-8
Cruiser (Light, Heavy, and Battle) 9-15
Capital (Battleship, Dreadnought, Super Dreadnought) 16-24
Super Capital (Monitors +) 25+
This allows for some overlap - a Heavy Frigate might be the same size as a Light Destroyer for example. And not everyone might have a Size 5 Frigate for example. Or use Frigates at all.
Now, I was basing them a bit on the StarFire order of categories. (Their actual order of ships, smallest to largest is: Explorer, Escort, Corvette, FF, DD, CL, CA, BC, BB, Super Dreadnought (yes, Dreadnought is missing), and Monitor.)
Another set I have is: Gunboat (1), Corvette (2), FF (4), DD (7), CL (10), Medium Cruiser (12), CA (14), BC (16), BB (21), DN (27).
I've also experimented with size categories from other games; Star Fleet Battles (though the variants and lack of concrete sizes made that a nightmare in spreadsheeting), Star Empires 2 (or 3 or 4?), even the old Alternity Warships supplement.
The other complicating factor is that SU in any version of Starmada isn't linear. A Size 6 ship is ~7.88x bigger than a Size 1 ship. A Size 10 is 15.63x.
And the end of the day I think the consensus (well, me and mj12 anyways) is pick something, go with it. Everyone's going to have different opinions. Which is why I settled for the 3 class ranges rather than break it down further. Though it might make for a nice suggestion* in the yet-unnamed Unity Supplement we're all dreaming of. 
*Dan, I don't care if you use it or not, all I ask is that if you do you simply credit me by name/alias for the idea (and don't be like another gaming company who took my forum posts, published them, and denied I ever made them).