Mardajam...the goal is always to get as many amp hours as you can out of the space available...in 3 dimensions. From your post, I assume you have space for only one 12V deep cycle battery...so just make sure you get a REAL deep cycle as big as will fit...not a dual use type battery (one that says deep cycle but on the label also talks about starting or cranking power). Measure your available space and then go to Trojan or Deka Marine to get the battery that fits and has the largest 20 hour rating in amp hours.
Markets | Trojan Battery Company
(just the models DC24/27and 31DT)
Good luck and welcome aboard.