I have the petco 6.6 bookshelf tank. I personally can't image a 5 inch frog living in it. Once you add the gravel and deco your probably looking at about 5-6 gallons and about 7-8 inches of height. You could always go with African dwarf frogs which would do much better in a short long tank.
I already have the frog and since he's a male he's gonna max out around 3". I've had him for 2 yrs in a 5 gallon. He seems perfectly healthy, just time for an upgrade. For the record, the 6.6 would be nearly twice as long as the tank he's currently living in.