Depends on what kind... Egyptian uromastyx get the largest getting around 2 1/2 feet. Mali uromastyx are a little larger then half that size i think(?). The Mali are the way to go cuz they are a reasonable size and are considered best type of uro to keep in captivity. So i would recommend a mali over the larger Egyptian.
Plus, since they are roamers... space is key. The larger the lizard, the larger the cage. Mali would need a 4ft cage while a Egyptian would need a 6ft one.