On a TRX, maximum width you can legally run is 1" wider than standard, so 7" is the maximum width you can legally run.
On any other U12, because they came with 5.5" width wheels from the factory, technically you can only run 6.5" as the maximum, but you can be sneaky and grab a tyre placard sticker from a TRX and stick it in the glove box, replacing the original one.