OEM LS mesh wheels are 15x6 and I'm pretty sure the factory tire size is 195/55/15, but people run 205/50/15s on the OEM wheels.
OEM steel wheels (hubcaps) are 14x5.5 and the tires should be 195/60/14
If you're looking to upgrade wheels without having to roll fenders, the biggest I'd go is 16x7 with anything from a +35 to +42 offset. Tires to match those would probably be 215/45/16s (guessing), maybe a 205/45/16.