All depends on what style of wheels you like, how low you want your suspension and if you are willing to do any arch work.
With the right offsets and tyres you can fit any size wheel from 13" to 17" any any width up to 9".
15x8 will fit with any offset from +35 to +10 any lower and they'll poke too much. 15x6.5 wheels will be a little thin but will work within any offset from +40 to 0.
The PCD (stud pattern) is 4x100 but with adaptors you can fit any wheels. Porsche wheels are 5x130 & you can get adaptors for them easily. 4x100 - 4/5 X 114.3 (common JDM wheel PCD) are available but they're expensive.
If you go wide wheels & low suspension you'll need arch work & stretched tyres.
Also you'll need think what your priority will be, good stance or good handling. Any wheels over 17" will ruin the handling & ride. Smaller wheels will look shit with a stock ride height and big wheels will require a lot of work with lowered suspension.
At the end of the day it's all down to personal preference. Get yourself onto the forums & have a look at the pic threads, work out what style you like & go from there. The MK2 has been around for so long that virtually every wheel size & width will have been tried.