There are two forms of 15 gallon tanks available:
Standard 15 Gallon - 24x12x12
High 15 Gallon - 20x10x18
There is no long per se, unless you're refering to the 24x12x12 dimension one. I would no recommend going less than 12" high, because it will limit you as to what type of equipment, plants, etc you can have.
Though if you can get a 15G in 24x12x12, you can get a 20G in 24x12x16.
More is better right?
Here is a
link to All-Glass aquariums detailing some of the common models they offer. (Select Standard from the menu)