How to measure
Finding the right sized bike is all about inside leg measurement.
This is easy to do:
1. Stand the rider, in shoes, against a wall
2. Place a small book between their legs, as high as is comfortable
3. Ask the rider to move away carefully and measure from the top of the book to the floor.
Tip: If they're particularly wriggly, mark the wall at the highest point of the book before asking them to step away.