Depends on what you're looking for. Lightweight, stopping power or just replacing the existing rotors.

A larger rotor will give you better stopping power, but weighs a little more. Better for downhill. Smaller rotors are better for cross country where you're brakes aren't under as much pressure and you want to scrounge some weight off your bike. Larger rotors are easier to bend and probably wear your pads out a little quicker.

I use 185s which is a bit of an overkill when i'm on the road, but it compensates for my riding ability when going downhill single track.

Your existing rotors (if applicable) will have the size written on them.

Depends on the type of bike, and type of riding.

First check what your frame and forks maximum rotor size is on the bike manufacturers website.

Then consider your style, if you do fast downhill racing, the larger the better. If you want a light weight XC racer, the smaller the better. For all mountain and general use, a mid size like 110 is very good.

You just need to change the adaptor. Did the same thing with my Elixir CRs

Depends on what type of riding you like and how much you want to spend.

I run Elixir CRs on all my bikes because they're reasonably lightweight and have good stopping power. They look good as well. I got a good price on them, otherwise i would have just gone for the Elixir Rs.

The new Avid XXs look good, but you'll have to sell your house to buy them. Check out MTBR at

http://www.mtbr.com/cat/brakes/disc-brake-system/PLS_1507crx.aspx

Just remember though if there is only 1 review on an item, the results aren't really that statistically reliable.