As per above, if only running the kickstart then you could possibly even get away with a very small battery indeed - I'd recommend a 3Ah unit as I've run that in the past on a bike with full road lighting and had zero problems.
You could conceivably get away with no battery at all - change all your turn and brake lights to LED's to reduce current draw, and use the stock headlamp unit. You may have to adjust idle up perhaps a few hundred rpm but that would be the only problem that I could think of.
Personally, I'd stick with a smaller than stock battery...or stock spec if you're using the electric start. The 3Ah unit is only slightly larger than a 20 pack of cigarettes so its pretty easy to hide.
Cheers - boingk