Re: Pick a face in document, pick same face in familydocument - how?

First, yes it is possible to select a face and get it geometry and information from it via Revit API.

But, I think you are new in the Revit API. So, I suggest you start learning the basics.

Here are some blogs for learning Revit 

Autodesk Revit Developer's Guide

Building Coder

AEC Developer Blog

And I strongly suggest you look through the Revit SDK samples.

Finally, I want to show you how to select a face and it is really easy.

Reference reference = uidoc.Selection.PickObject(ObjectType.Face);
Element e = doc.GetElement(reference);

GeometryObject geomObj = e.GetGeometryObjectFromReference(reference);

Face face = geomObj as Face;
double area = face.Area;

I hope this helps,

Have a good day,

Recep.