Do I REALLY NEED a tall cage for a pink toe tarantula?

I am getting a pink toe tarantula from a local pet shop for $30, originally intending to get a cobalt, but I wasnt experienced enough. So the terrerium I have is not a tall one, and has a screen lid, so if I put it on it's side, the substrate will come out of the screen. It is a 15 gallon, 20 inches long, and 12 inches tall. So, do I NEEEED to have a tall one, or is this okay?PLZ HELPP!