ANSWER:

Arms must be fully covered upto end of the wrists. And she must be fully covered with a loose outer garment that covers her entire body except face and hands. 

When praying outside in public or in front of non mahram males she must keep her face covered as well if she wears niqab. 

https://youtu.be/6zSOyAW4-VY

How much hands should be covered during Salah for women? And what other to be covered as a woman??