Go at least 15 gallon. If you know anyone that can custom make one, you could get one built for your specific needs. I just finished building two, one for myself and one for a friend. Custom building is nice because you can choose the size you want, to make it fit where you need it to go. Mine measures 20"Lx16"Wx13"D, and will hold between 17-18 gallons. I wish I would have made mine a little narrower so I could put it in the factory location (kind of), and just raise it up alot from factory, but it is a little too wide and would hit the driveshaft (might can make it fit though, haven't looked at it too close).
For reference, my friend spent $100 on a 3x12 sheet of 1/8" aluminum (thanks FishHunt), and that was plenty to my both tanks, and I have some leftover I am going to use as a floor in the 'bed' of my truck.
You just have to find somebody that can TIG weld aluminum, a spoolgun WILL NOT work, the weld it WAY too porous.