A wide variety of sizes will work. Its not critical. If the juvie is 3-4 inches, I'd start with something like a 5 gallon cube. 12x12x12". If its already larger than that, I'd just go to a 10 gallon tank which is 10x20". No harm if you go a little smaller or little bigger than either of these. My tiny sling has grown to nearly 3 inches, and he/she is still in a 4" AMAC box. Planning to re-house it soon.A juvie