I am looking for a way to get the sheet metal flat pattern face and the perimeter of that face.

So far I have the code to put the part in the flatten state.

Set swDocExt = swModel.Extension
'get Flat Pattern feature
boolstatus = swDocExt.SelectByID2("Flat-Pattern1", "BODYFEATURE", 0, 0, 0, False, 0, Nothing, 0)
'put in flatten state
swModel.SetBendState 2
boolstatus = swModel.EditRebuild3
'return to bent state
swModel.SetBendState 3
boolstatus = swModel.EditRebuild3

Now I just need to figure out how to select the face and get the perimeter. 

Dim instance As IMeasure

Dim value As Double

value = instance.Perimeter