Standard buggy wheels are 2.2" diameter, and all buggy tyres are for 2.2" wheels.
If you want bigger, then 1/10th truck wheels such as the
HPI Super Star will let you fit bigger truck tyres. These use a 12mm hex which are found on quite a few buggies. You can also fit short course truck wheels on many buggies.
If you want to know what's possible, then if you run a buggy with 12mm wheel hex drives you can find wheel adapters that will let you fit anything up to the 3.8" Proline Desperado wheels, although sorting out gearing for 7 inch diameter tyres will be a bit tricky.