11" wide!
i had those on my camaro with 335 tires
here's a good page:
http://wheeldynamics.net/appguide.html
Boxster:
Can use most wheels from the 996 with spacers in the rear. 40mm-65mm et front and 40-50mm et rear (Stock 18x10 is 47mm) the 18x10et 65mm 996 wheel will require a 15mm spacer to clear the shock.
Most extreme is the Techart 18x10.5 et 42mm wheel for the rear. Use zip ties for the parking brake cable. 285/30/18 tire is about the largest you can use.
Can use 19' from 987
Most extreme fit 20x9 et50mm and 20x10 et 60mm(15mm spacer) with 245/30/20 and 285/20/20
20x9et 50mm and 20x10et 60mm below
996:
On the 99-01 cars you can use up to 11" wide rears with an offset of 52mm. The factory 997 wheel in 19x11 offset 67mm will need a small spacer in the rear with some tires. The 2002 and up cars do not have this issue as the track was slightly widened. GT3 is 18x11 offset 63mm.