For most males that T8 would be fine. I know that I would not be comfortable using a 20 gallon tank for life.
I have a 5 foot girl in a a tank that is 6 inches taller than the T8, otherwise it's the same. But I have plans to move her to T12 equivalent, 48x24x24. She loves to climb, so I'm going taller than the norm.
The clutter on top is to keep the cat off.