Here is a list of SLA type batteries by size and voltage among other specs.
SLA batteries from 1000bulbs.com
http://www.atbatt.com/sealed-lead-ac...es/b/yuasa.asp
Looks like a list of SLA batteries with dimensions as well. I would try one of these or Google it and see if they are working ok for people. I have read that people use them in bikes but they just have fewer cranking amps. I'm not sure about the electric starter performance with these but I know the bike would run just fine and the battery would last good. They are an AGM battery so you must use an AGM compatible charger. The
Genius 1100 is the answer for that one. It does AGM batteries which includes Oddessy and Optima batteries both 12V and 6V. This 1100 is a hell of a deal and with all the features it has blown away my other trickle and tender chargers I have used - the "tender" one and the Yuasa tender which isn't anything special. I have actually brought back many batteries from the dead and I actually found a battery being used as a gate door stop at a house and brought it back to like new condition to use in a generator I have! My friends thought there was no way that one would come back but it did after a day of charger conditioning from the Genius Charger. Pretty amazing. I am ALWAYS charging batteries in cars, trucks, motorcycles, generators and a forklift haha. Genius Chargers are a must. You can get them straight from Amazon.com the cheapest I think.
Save money and buy a super affordable AGM and then buy a Genius 1100 charger to compliment it and you should be very happy.
Cyclegear.com also sells tons of batteries. You can use their site for reference as well. They have a good range of batteries and they will help if you call them up I'm sure.