i have a 2007 2500hd and i am getting either a 4 inch or 6inch lift does anyone know how big of tires i can put on the 4 or 6 without rubbing...i want the biggest tire possable  

According to this months issue, it would seem that with a 6" lift on your 07 a 35" tire would be the biggest you can go without rubbing.

(Applying Dirt Logic - Page 40 - June '07)  

A lot of the lift manufacturers claim you can run 35's on a 6 inch lift but do some research before you make the final purchase. Rough Country claims their X-Series 6 inch kit clears 35's but I had to do some significant valance trimming to clear 34's (305/70R17)and that was with my torsion bars cranked almost all the way up! Check out CognitoMotorsports.com they specialize in GM lifts and accessories. I've talked to their techs, they know what they're talking about and very helpful! Good Luck.  

I have a good friend with a 6" RCD Kit on his 2500 HD (not an 07 though) and it clears 35's with plenty of room to spare...  

My question is what is it? classic body style or the new silverado, because the new silverado has smaller whell well openings!  

I have a 2004 GMC 2500HD with a 6-inch Fabtech lift and I'm running 315/75R16 (35's) and I have to problem with rubbing.  In "my" opinion the 2500's have more clearence in the wheelwell area then a 1500.  That being said I really would research it before any big purchase.