Decide which age bracket you fall into eg. If your child is 7 you should need a size medium. If your child is 9 you may want to go for the next size to allow for growing room.
If your child is bigger or smaller than average you may need to choose the blanket size up or down .
If you are still unsure, measure while sitting the distance from the toe to the knees to under the arms. Measure in a straight line. Don’t follow the curves of the legs. You should have 2 straight lines from toe to knee and from knee to under the arms. This will be a good guide for choosing your blanket size.