650B = 584 millimeters = 23 in.
26-in = 559 millimeters = 22 in.
WHEEL diameters depend on the tires, but range from about 23-inches to 29-inches. Unless you include the ultra Fatties that go up to about 31 inches.
650B = Medium = 27.5-inch. A popular size in France in the '40s, recently came back and is solidly locked in (a good thing).
26-inch = Small = never actually had an equivalent reference to "700c/650B". Always just 26-inch.
WHICH SIZE FOR WHICH USE?
It's not that tricky or specific. Here's a good-enough rough guide that only a true internet weirdo could find fault with:
700C is most common for:
- Racing against other 700c riders, and with teammates who also ride 700C wheels. If you need a new wheel, you’ll be able to get one.
- On any bike that doesn’t require compromises in frame design or clearance (which, of course, is part of frame design, but I want to emphasize the clearance part) by using the Big wheel.
- Big mountain bikes. The bigger wheel rolls over bumps better. If you doubt this, consider the difference between riding over a pothole or a driveway lip with a skateboard (which has small wheels, don’tcha know), compared to riding over the same thing with a bike with a much bigger wheel on it. The difference between Big, Medium, and Small bicycle wheels isn’t as dramatic as that, but the dramaticness makes the point, and the same physics (is that what it is?) applies.