A 40 may work initially, but remember these guys have to have a basking spot of upwards of 120 degrees, so you have to figure, if I have a heat lamp on one end of a 40 gallon making a 120 degree basking spot, how hot is my cool end?
I would recommend planning for AT LEAST a 4x2x2 enclosure, if not a little bigger for a larger uro. The bigger always the better. Uros aren't like a BP they want some room to move around. If your familiar with bearded dragon care, the enclosure recommendations are typically the same.