Can of worms!

Ok I'm a roadie so I don't profess to know much about MTB perhaps Jamie can add something.

But 29er isn't the standard they are still pretty niche I think they are for more open terrain when you want to blat along pretty quickly. I'm thinking upland moors etc. it's the same sized wheel as road bikes I think.

But the standard if there is one is still 26" which is best for the technical trails.

650b is half way between the two so professes the best of both, or worst of both depending on your point of view!