And of course in the end I go a whole different route. I decided rather than file size I would go after a specific set of workbooks, those that are saved as our "source" workbooks, most of which are huge, all of which are protected so we can't save changes, so no reason to have autosave turned on for them. Amended code is below. 
Private Sub app_Workbookopen(ByVal Wkb As Workbook)
'Check out autosave settings for Source files I open
'Only look at the source files folder
If Wkb.Path = "\\12aust1001fs01\SHARE10011\Budget\SOBUDGET\_Protected_Data\Source_Docs\_OOE_MOF_PivotTables" Then
    With Wkb
    If MsgBox("Autosave is " & _
              IIf(EnableAutoRecover, "ENABLED", "DISABLED") & " for " & .Name & _
              IIf(EnableAutoRecover, ".  DISABLE?", ".  ENABLE?"), vbYesNo + vbInformation, "Turn Autosave On or Off") = vbYes Then _
       .EnableAutoRecover = Not (.EnableAutoRecover)
  End With

End If
End Sub