I see a lot of people trying / considering to put 650B wheels on 26” mountain bikes and 700c road bikes. While I understand that this is common to do to just find out more, but it will never be a really good answer as the handling of such bike is not really what a proper 650B bicycle will be.
Why? First off, BB height will not be correct / optimal after the conversion. Some may ask, what is the big deal. To me, it is a big deal. Without a proper BB height for the designed use, weight balance of rider over the bike will not be optimal, for most 26” mtb frames, it will be higher while for 700c road frames, it will be lower. Then, there is the front end geometry to worry about. With a different wheel diameter in front, combined with unchanged rake of your fork, trail will end up bigger or smaller. The result, again, will not be optimal. Of course, depending on what tire width you chose for your 650B, it will also affect the contact patch size. Lastly, chainstay length. Some believe shorter is better, but I disagree with this notion. Chainstay length should again be application specific and it is a factor within a whole pool of factors for designing a frame.
I think 650B is best suited for riders of certain body specifics. Height, weight, riding style, and application. In fact, that is why 650B had survived in Europe all these years. During the popular cyclotouring era, 650B was the wheel size of choice. Because of application and road condition at that time, it really is the best. Some riders who carry on the cyclotouring sport have chosen 700c with 30-ish tire size in modern times but I can tell you this size is not optimal for a lot of riders.
As for off-road, 650B has its place too. Kirk Pacenti will tell you one of the main reasons is for long travel full suspension bikes, and I completely agree with that. On top of that, it is also very nice for hard tails. As you might have read that many people have switched to 29ers over the past few years, and most of them will cite the better momentum, how much easier it is to roll over obstacles, etc. These benefits will also appear on a 650B bike, not in the same magnitude, but the characteristics is there. Also, bear in mind why some 26” riders tried 29 and did not change. They will tell you cornering is just not very natural, slow technical handling is cumbersome and that their climbing prowess is not as good. This is where 650B will shine just like a 26” mtb plus some of the benefits of 29er. But these are simply generalization. Like for road bikes, the choice should be rider driven, their body dimension, riding style and application.
I believe riders of 5’4 to 5’9 are major benefactors of this wheel size. You will have a bike that handles properly, no toe overlap, fit properly with correct weight balance. As tire and rim choices expand, we will see more discerning cyclists switching to 650B wheels, and purpose built 650B frames.
So, my suggestion is go ahead and experiment, but a really nice bike is most likely a custom made to measure bicycle, consult your favorite framebuilders and allow them to give you suggestions, not just on wheel size, but also on fitting and frame design. Also, visit the upcoming North American Handmade Bicycle Show (NAHBS) in Portland, Oregon. I hope to see you there.