If you look to the immediate right hand side there is a bud light package, looks like a 24 pack case.
Diagonally there is the end of a sofa, so I would guesstimate about 2x2 or 24 inches square or above.
It could house a hognose, garter snake, milk snake or other small colubrid very comfortably.
Making a cover out of plywood is a breeze, and a bit cheaper than buying big screen tops.
With a small colubrid, a basking spot would be really all you need for heat.