[Grant@Tirerack]

The biggest tire sizes we have test fit here are a 255/50-17 or a 235/50-18. You can run up to a 235 without rubbing in the wheel wells (even lock to lock steering) but the 255s are very tight and may rub when parking etc... The guys running 20" are typically running underload tires or are using wheels that are offset so far that they are replacing wheel bearings on a regular basis. The hubs just can't handle those offests for long. It puts too much stress on the bearings. The other issue is weight. Most 20" wheels/tires are going to weigh quite a bit more. The brakes on these earlier trucks are not going to be able to handle the added stress. That's why you see these guys buying the big brake upgrades as well. It takes more brake force to stop that much weight from rotating. For most situations, I would suggest sticking with a nice 17". That's about the maximum the Blazer can handle without a lot of work and $$$.