It depends on what the width and height of the tank are. I've been shopping for a 3 ft long tank lately and these are the sizes I've come across so far -
Measurements are length x width x height
30 gallon 36" x 12" x 16"
45 gallon (high) 36" x 12" x 24"
40 gallon 36" x 15" x 16"
50 gallon 36" x 15" x 20"
55 gallion (high) 36" x 15" x 24"
55 gallon (regular) 36" x 18" x 20"
65 gallon (high) 36" x 18" x 24"
30 gallon (breeder) 36" x 18" x 12"
40 gallon (breeder) 36" x 18" x 16"
I'm sure there are a few more size variations available, these are just the ones I've looked at in the last couple of weeks.