I actually won't need too big of a tank when these guys are full grown. A 75g will probably work just fine. The way I keep fish is a little different than most people... I like to "overstock" a bit.
Edit: I just thought I'd add, a 75g would probably be all I NEED. I am aiming for a 120g before these guys max out.
This is really the only tank I have like this. My other tanks are what you consider more "traditional". This one could be considered JDM-inspired.