Making sure that faces have the same dimensions this is my very first try at using Blender so sorry in advance for the noobish question.

I have this very simple object (supposed to be a window): 


I'd like to find a way to be sure that face A as the same width as face B (and potentially top and bottom faces too).

I created this object using extrusion on a simple cube... I don't even know if I was supposed to this that way.

Thanks for your help and guidance.
I went with @moonboots answer (all glory to him).


Delete the group of faces that A is composed of
Duplicate the group of faces that B is composed of
Snap that duplicate at A's position