What is the largest that we could make a wheeled or tracked vehicle with modern technology?
Giant powered axle
Build two copies of the world's largest ferris wheel, each at 550ft high. Remove the cabins so they can roll on the rims. Attach them together with an axle long enough so they don't tip over (maybe 400ft). Then suspend a propeller aircraft engine from each end of the axle. You can steer like with a bulldozers treads: activate just the left engine to turn right, just the right to turn left, both to go forward. Prop engines can go in reverse too. Finally hang a cockpit from the middle of the axle.
It's big. It can probably be made to cruise over salt flats. Likely not good for much else.
Could you get something bigger in theory? Not sure.