swiss ball size
 

What size should a swiss ball be in proportion to yourself?
I'm quite a tall guy, 6'5" or so, do I need quite a large one, or would a regular size one do?
as well, how blown up does it have to be?

Go to the store and read the boxes. They will tell you the size and correct pressure for that particullar ball.

the more air the ball has in it, the more difficult/beneficial the exercies will be. you should probably have a larger ball, you can still make do with something a touch smaller.