Ok I finally got some tires for my boxster rims. I knew they would need some spacers in the back. My question is does anybody know the size and the length of stud to go in the hub.

Here's the front. I think I will polish the rims down the road.

you need AT LEAST 1x the diameter (14mm) thread engagement - 1.5x is better.
get studs long enough to achieve that.
the longest studs i've seen generally available are 100mm (4"). if you need more than 4" of stud for your spacers, you'll need to come up with an alternate plan ...

long studs are a standard replacement part - all The Usual Suspects have them. shop around - the prices seem to vary wildly based on nothing i have ever determined ...