I started off with a 22.5Ah back-pack battery, but a 6Kg or so block of alloy sitting on the base of my spine was a bad accident waiting to happen where we ride, so I had Jimmy at BGA Reworking (aka Insat Intl) build me a battery in a Topeak Large saddlebag and he squeezed 11.6Ah in there (we now have 4 of them they're that good)...here's details of the bike and if you look in the S-peds forum there's more of my builds...
http://www.pedelecs.co.uk/forum/threads/2004-s-works-enduro-bbs02.23363/
http://www.pedelecs.co.uk/forum/threads/2004-s-works-enduro-bbs02.23363/