|My Big Girl on a new-to-her 24er mountain bike.|
When I was thinking about moving her up in wheel size, I estimated 24" wheels for a kid her size would be roughly equivalent to a 29+ for me. To examine this idea, here is a little math. The tread diameter of a 24" x 1.9" tire (61.3 cm) divided by her height (134.6 cm) returns a ratio of 0.455. For comparison, using the tread diameter of a 29" x 2.3" tire (74.2 cm) divided by my height (187 cm) is a ratio of 0.392.
Therefore, if I were to ride a bike with the same tire diameter/height ratio as hers, it would have a rolling diameter of 85.16 cm, or about 33.5". If applying the ratio to tire width, the 1.9" tire on her bike is equivalent to a 2.6" tire, if scaled up to my size. With 2.2" wide tires, she'd have the kid-size equivalent of Surly Knard 3.0" tires. Of course, as she grows the ratio will shrink, until the 24" wheel diameter will eventually feel small for her and we'll look to a 26er.
Anyway, for the 3 of you who read this far, I hope you were entertained. In any case, this site has a strict no-refund policy.