If your going to keep the electric start, then you'll need the stock 12V 14 amps, you might be able to go down to 10 or 12 amps. If you go kick-only, then you can go real low, like 3 amps with minimal lighting. I run a 5 amp battery, kick only with all low draw LED lights on by bobber.
Those 3 amp batteries are about the size of a pack of cigs, my 5 amp is about as big as a coffee cup.
Search the forum for Small Battery, there's lots of threads on the subject.