Hi
Adderall, like all stims is dosed by symptoms, not size, age or weight. I would give it a week or two to see if it helps, because as with any med, it is best to start low and increase.
FWIW my friend takes Adderall, she is a very tiny person and takes 50 mgs, while her brother is large, tall and muscular and takes 10 mgs. So every person is different.