I measure mine on the trampoline. She is perfectly willing to stretch out on a warm day and it is no big deal to put a tape measure next to her.
I have my girl in a 4.5' X 3' vision tank. It seems a bit large right now when she coils up on the heat. But when she is stretched from one end to the other, then back again, then part way back again, I wonder if it's to small. The vision holds the humidity and heat really well, she always has nice sheds, it is easy to clean and it is very secure. You might consider some sort of pre-fab cage as well. I got mine from craigslist for $140.