The converse of using big wheels is that they present a bigger target if mounted outside of the bodywork so you'd need to consider how you'd protect them.

The Honey Badger 2.x uses some RC car wheels from a 1:6 (I think) monster truck which measure some 110mm diameter by 40mm wide wheels, which are secured to the wheel hub by some M5 bolts. They have been thoroughly abused, way more than its intended application but are still in round and in one piece.

As an aside, it stuck me when I first saw your robot, it looks vaguely like a old Battlebots competitor, Das Bot from Season 1.0 (http://battlebots.wikia.com/wiki/Das_Bot).