Dimension Curtain wall Mullions

Is there a way to dimension the mullions of a curtain wall?
I know that you can get grid reference from the curtain wall but the goal is to dimension the mullions instead of the curtain wall grid(what I am trying to achieve: Picture below)

So far I have tried to get the faces references of the mullion but somehow element.faces is not working.

Any ideas/workarounds?

Thank you in advance