TTT,
On my RD350, with a Powerdynamo PMA/ CDI system, I use a Super B 2600 LiPoFe battery. Guess it is 2.6 Ah, weighs 450 g/ 1 lb, and is the size of maybe 2 packs of long 100 cigarettes. I have 3 LiPoFe batteries in my bikes. No lead acid for me anymore. If you go the same route, make sure you have a good regulator.