..what?
If you go by that rule, you're saying the tank should be 1ft long and 6" wide. Which is fine, but then you recommend a 20 gallon which is closer to 2ft long.
10-15gallons are fine for that size animal. If you're having trouble fitting everything, make sure the hides are snug and tight fitting for him and that the water bowl isn't much bigger than a ramekin. They don't need enough space to soak, just enough to drink.
http://www.amazon.com/Set-of-4-Ceram...6685225&sr=1-1