Smallest proper battery?

Im running Ants 200W PMA kit and a Bransden ignition on a kickstart only xs I want plenty of electric so my I can run a bright bulb in the headlight for night riding.

Whats the smallest sized battery I can get away with?

I'm using a small 12v 3AH battery, to fit under the seat hump, with exactly the same set up as you have.

The battery is a Leoch LTX4L-BS

http://www.ballisticparts.com/

Don't know if it's small enough for you but these batteries can replace a stock unit, they are small, light and have a lot of power. The 8 cell should work fine, they are lithium-ion batteries, I think they have a 4 cell version too.

I've got this:

http://www.ebay.co.uk/itm/150840263431?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1439.l2649

I have run a 0.8ah battery before. For about 2k miles. It was the size of a pack of cigarettes and I had a boyer high output 360 ignition. I swapped to a Shorai when I did the rephase and hughs charging upgrade.

The only thing I would say is that you shouldn't intend to run a high beam or horn. Both of those will kill a tiny little battery like what I had. Otherwise it ran great and was dirt cheap. ~$8. Bought two and always carried a back up that I never needed to use.