650b for small rider?
I personally am a fan of the 650b movement. With a 650x47b tire, you get approximately the same rollout diameter that you get with a 700x28c. So as far as the distance per revolution, you are about the same with a 650bx47 as you would be on most road bikes. The benefits start with increased air volume. This leads to a much softer ride.

If you are talking about a conversion from a 700c to a 650b on a bike, there are very interesting things that happen to the geometry. Your bb drop will be greater, so clearance could become an issue. with a lower bb, however, comes a lower center of gravity. This could increase stability through corners. Another consideration is that most bikes designed for a certain sized tire. By changing that, you could affect things such as trail. A decrease in trail would help in low speed handling, but could greatly hinder higher speed stability. 

One last consideration for converting a 700c to 650b is braking. If the bike has discs, then no worries. If they are v or canti brakes, then the reach may not be great enough to have the pads make contact with the braking surface on the wheel.

http://ridinggravel.com/components/wheel-comparison-650b-vs-700c/

There are some bikes road type bikes that are designed with the 650b in mind. Some are children's bikes, but if the rider is small, a children's bike could be easier to control.