Disc brake mounting questions

You can run either a 160 180 or 203mm rotor as long as you have the right size adaptor.
For the p2 fork you'll need a IS to post mount adaptor in whatever rotor size you decide to go with.

This^

The industry seems to be moving towards post mount fitting for both frame and forks. This looks like a good thing on the face of it (have'nt really thought much about it TBH). But most frames and forks come with IS mounts, so you'll need an adaptor.

This pic shows a IS to post adaptor.

If you want to run a 160mm front, you'll need one of these.
http://www.chainreactioncycles.com/Models.aspx?ModelID=20304
For a 185mm you'll need one of these.
http://www.chainreactioncycles.com/Models.aspx?ModelID=13721

I run 160 front and 140 rear. But thats a light weight handbag XC set-up. You may want to go to 185.

The 6 hole disc mounting is very standard so just pick the size rotor you want to run.