Use same amount of tiles (14400 if it's correct) which the basic square have but without hard limitations to the shape
There can be made some limit to ships from being too wide or too long if it can be exploited (it will be exploited, I know
). Make the building grid square bigger but with that block limitation.
Building grid itself also should be remain at least because it helpful in building process.
It's every time annoying to seeing multiplayer matches where ships in max size categories near to be square or circle/half-circle