Almost all battery boxes will fit a group 24.
Most will fit a group 27 and some will fit group 31 or 6-volt. If you just want to have a battery for the trailer brakes (required), then the smallest and cheapest 12-volt group 24 battery will suffice. A bigger battery gives you greater capacity for those times when you do not have electrical hookups.
By the way, most dealers will be happy to supply the battery or are you buying used?
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD