[gavinyuill]

It is normal to have a stagger in the wheel sizes, smaller on the front than the back (so I hope you have your measurements the wrong way round) in order to promote understeer, which is generally though of as safer than oversteer.

I would have thought with that stagger, your wheels will be different widths anyway, so you may not have the choice. On the boxster, you have a lot of understeer built in to the car, so running the same size would not hurt much but there is also little reason to do so.

In short - different sizes are good for safe(er) driving. However, take it to a tire shop or two and ask their opinion.