For anyone interested, I have found the solution I was looking for. Here it is:
Code:
Private Function setMedia(size As String)
Dim mediaNames As Variant
Dim x As Integer
mediaNames = ThisDrawing.ActiveLayout.GetCanonicalMediaNames
For x = LBound(mediaNames) To UBound(mediaNames)
If StrComp(ThisDrawing.ActiveLayout.GetLocaleMediaName(mediaNames(x)), size, vbTextCompare) = 0 Then
ThisDrawing.ActiveLayout.CanonicalMediaName = mediaNames(x)
ThisDrawing.ActiveLayout.RefreshPlotDeviceInfo
End If
Next
End Function
I set my plot device then run this function supplying the name of the paper size to use.