I've got a 29er Riba with standard XT brakes so I guess that is 165mm rotors and the mounts are international standard.

What do I need to do if I want to get a bigger rotor on there, it is just a matter of an adapter and a bigger disc or do I need to do something more complicated?

no bigger disc and adapter check the maximum rated size for your fork and swa[ pver. Make sur eyou know whther the brak enad mounts are IS or post

Brake is IS mount. How do I know what adapter to get?

What type of mount is on the fork?
What type of mounting on the caliper?

If both are IS then this will allow you to run a 185mm disc

Various adaptors here depending on the combination

http://www.chainreactioncycles.com/Categories.aspx?CategoryID=563

Follow up question:

I ride/race XC but I am about 15 stone – hence wanting more stopping power. Should I get 180mm or 203mm rotors?

I'm heavier than you and find 180 front/160 rear more than enough.

personal choice 203 is bigger has more stopping power but also a weight penalty.

I think I'll probably go for the 180s, just thought I'd ask.

Somehow it seems odd to put 203 on a hardtail 29er XC race bike anyway 🙂

Now to find out about that adapter…

Just an aside, if you're fitting a bigger disc its probably worth being a bit more rigorous cleaning/servicing the fork.

When I eventually wore my Rebas out, it was on the disc brake side (I like stoppies…) so I imagine a bigger disc/bigger wheel would accelerate this. Cleaning and re-lubing the lowers on Rebas is even easier than on foxes, so its worth keeping on top of.

I was considering servicing them but I haven't managed to find any suspension grease yet (judy butter) and it is a very easy job to keep putting off…

203mm Juicy Threes on my Rebas with no problems.

I run 203mm Shimano rotors on both my 29er Reba forks with no problems. i weigh the same as you. I have 180mm on my Rigid SS which seems ok but i still prefer the biggest discs i can fit on generally. That's a personal choice though.