I have a 1974 Sovereign
and used a Group 24 maintenance free battery. You could probably go a bit bigger if you needed to.
Other group sizes that are about the same size as a group 24:
(Group 24 = 10 5/8" x 6 3/4" x 8 3/8")
Group 34 = 10 3/16" x 6 7/8" x 8"
Group 78 = 10 3/16" x 6 7/8" x 8 1/8"
My advice would be to go with a maintenance free battery and avoid having to top up with water, but more importantly, avoid the risk of acid spills and the corrosion that goes along with it.
I would also recommend a battery designed for cycling rather than starting - they cost more, but will keep going much longer when you are using the trailer without electrical hook-up.
My choice would be one of the new Exide Extreme Duty Orbital batteries as they have excellent cycle life and a very low internal resistance so you can charge them quickly and also get a lot of power out of them if you need to. You can probably get these at NAPA but they might be 'own-labelled'!