I would like to know what are the biggest wheels you can put on the STS and if anyone has had any problems with the Sensors on any aftermarket wheels????

Already several threads on this (Wheel/Tire sizes). Probably best to run a search for many, many comments already posted.

As for the TPMS sensors. I do not recall anyone commenting on issues and there are quite a few that have talked about various Wheel/Tire combo's that use TPMS.

According to tirerack.com, you can go up to a 20" wheel. Just feep the size ratio the same front and rear or you will have problems with your traction control

i seen up to 22's with 255/30/22 all around. i also heard you can fit up to 305/30/22 in the rear but i dont now if its true.

42s.