If it's 8" high, you can use that size, for a male, forever.
I keep all my adult males and females in 28 qt tubs, which are just under 24"d x 17"w x 6"d. You might want to get a tub that's a little deeper than 12" in about a year, but that surface area is about the minimum for an adult male.
What are these mojavas I keep hearing so much about?