First, measure the distance between the front wheel and the rear wheel, as shown above. Then, from the chart below, determine which size is for you.
| size | distance |
|---|---|
| Small | 8 inches to 8 3/4 inches (20.3cm-22.2cm) |
| Medium | 8 3/4 inches to 9 1/2 inches (22.2cm-24.0cm) |
| Large | 9 1/2 inches to 10 inches (24.0cm-25.4cm) |
| X-Large | 10 inches to 11 inches (25.4cm-27.9cm) |
For example, if the distance that you measure is 9 inches, then you would need to order the Medium size unit.