Okay, so to fit 8s, 9s or even 10s, you'll most probably have to roll your arches and in some of the more extreme cases you will have to cut and pull the arches.
The Wheel's ET, tyre size / stretch level and Camber settings have also got a lot to do with it.
A quick way of finding out, would be to find the ET of your current wheels, then measure the distance from your hub mounting section of the wheels to the edge of the arch.
the extra space you have is the extra wheel depth you can have.
Now, if you want a bit of dish or lip on your wheels, you need to go for a low ET set up, something like ET15 or ET20...(standard 15" mini one wheels are ET45 for example)
if however you are not going for deep dish wheels, you need to get a high ET wheel and probably use spacers to avoid hitting the inside of the wheel arch or your suspension struts.
Hope this helps