Sheldon makes me laugh.

Taken from Sheldon Browns great website! So much useful info. I believe that he passed away a few years ago - so never got to see the rise of a new wheel size.

Twenty Niner
The so-called "29 inch" tire size is not actually a new size, it is just a new marketing term for 622 mm (700C) wheels with fat tires.
This size is probably a good option for taller riders for off-road use, compared with 559 mm (26 inch decimal) wheels. Larger-diameter wheels don't sink as far into holes, so they give a better ride in rough conditions.

For shorter riders, wheels this large will cause unfortunate compromises in frame geometry, so they're probably not a good option for riders of short or medium height.

Twenty-seven Five (27five)
Idiotic, confusing marketing buzzword referring to the 584 mm (650 B / 26 x 1 ½") Please do not use this bogus designation, let it die a well-deserved death. Tire-sizing is confusing enough without making up new designations for existing sizes.