To give you an idea on size for ship battles, one of Paizo's flipmats measures two ships in a battle at 16 usable squares long (80 ft) and 20 usable squares long (100 ft). They are both about 6 usable squares wide (30 ft) at the widest point. If you plan on using them as props with cannon fire and raiding from one ship to the other, your map might be sized just fine. If you mean for the ships to actually be able to move around each other, that map would be way too small. Just depends on how you want the ship battle to go.
So far as the view goes, I like how you've laid out the amphitheater. I think it gives a pretty good feel for what we're looking at while leaving ample space for the arena itself. If you're going to change the size of the arena, I'd suggest just changing the grid size. Making the grids about 3/4 of their current size would probably be as small as I'd go on this map though.
