I'm not familiar with this particular model. The manual on mesa's website references that it can push 100 watts, but I don't know the first thing about green stripes from purple stripes. With that said, I do know a little about speakers.
You have to account for watts and ohms. Watts reference the amount of power the speaker(s) can handle. The rule of thumb is that you want to have enough watts to cover what the amp can push out. Since the mkiii seems to push up to 100 watts(please confirm this point as a fact), so you would want to choose 2 speakers that can handle at least 50W each as each speaker would split the total 100watt load. You may safely choose speakers that can handle more than 50 watts but you should be cautious about choosing speakers that handle less.
With Ohms, it has to do with how the speakers are wired. You have series wiring and parallel wiring. 2 8ohm speakers wired in series has a 16ohm load (8+8). 2 8ohm speakers wired in parallel has a load of 4ohms. Mesa amps tend to have 8ohm and 4ohm outputs. Marshall amps tend to have 16ohm and 8ohm outputs. The common ground is 8ohms. If you want to have versatility, it makes sense to get 2 16ohm speakers and wire them in parallel for a total load of 8ohms.
Hope this isn't too confusing. There is lots of stuff on the subject. Read up and feel comfortable. Once you understand the principles, the real problem will be figuring out what speakers best suit the sound you are looking for.
Good luck
