That size should be fine.
I would fill it about 3/4 full of washed playsand. I use the one produced by Kings that comes in a white bag with red, blue, yellow sandbox toys on the front of it. That one has never caused an impaction or infection with any of my chameleons. It should be moistened just enough to hold a tunnel. I don't pack it down.
Add a branch so she can get in and out easily.