Is it true that when you get braces and you have a long face your jaws widen up so your face looks normal?

Braces. The widening of the face is limited by braces alone. Braces will help straighten teeth but braces cannot change bone width. If you need a wider face then you may need an expander or even surgery depending on your age. The smile can be broadened a little with braces but to the boundaries of the existing bone and no further.
Braces. Your face does not change. The braces adjust the position of teeth so you can have better occlusion (close bite). Your jaw may change if an oral surgeon does jaw surgery (if you have retrognathia- jaw is back or - if you have prognathia: jaw is foward). Again the goal is good teeth occlusion. Braces does not change your face.