[Suicide Junkie]

Regarding the hull sizes:
I fully endorse using a system in which
A) Build cost = Size ^ 2
B) Total maintenance cost = Size^0.5

The practical result being that you can maintain 11 "size-1" frigates (total tonnage=11) at the same cost as 5 "size-5" (total tonnage=25) battleships.
But you can build 25 of those frigates for every 5 battleships.
Losses are quicker to replace with small ships, but you can field more tonnage if you spend the time and money on bigger ships.

As a bonus, you can provide all the hulls without tech requirements and still be confident that all of them will be used at some point during the game by the players.