The basics:
The smaller the icon, the smaller the ship!
If an icon has a line beneath it, it's a more hurty version of that ship!
If an icon has a pointy top, it's more likely to hurt you!
If an icon has a blunt top, it's less likely to hurt you!

For pointy stuff (=combat vessels):
Edges = major ship class
Bars = next minor ship class
3 edges = frigate
3 edges + 1 bar = destroyer
5 edges = cruiser
5 edges + 1 bar = battlecruiser
6 edges = battleship