It depends on the space you have available in your battery box. You should have either a group 24 or group 27 battery. I would suggest buying an AGM battery (Lifeline brand) as they require no maintenance or liquid, hold their charge much longer than liquid cell batteries and re-charge much faster. You can also put them inside the trailer as they give off almost no gas.
You can also double up with 2-12V batteries in parallel for double the amp-hour capacity, or you could use 2-6V golf cart batteries in series. They will give you about 220 amp-hours and about double the charge-discharge cycles that a 12V