Here is an open macro that automatically re-sizes the zoom based on users
resolution. It is set to do each sheet based on the defined range to be
used. So, in THIS case, each sheet must have the defined name sheet1!myzoom
, etc.

Private Sub Workbook_Open()
Application.Calculation = xlCalculationManual
Application.ScreenUpdating = False

   Dim tstRange As Range
   Dim curWks As Worksheet
   Dim wks As Worksheet

   For Each wks In ActiveWorkbook.Worksheets
           With wks
           If wks.Name <> "Parts" Then
           Set tstRange = Nothing
           On Error Resume Next
           Set tstRange = .Range("myzoom")
           On Error GoTo 0
           If tstRange Is Nothing Then
           'do nothing
           Else
               .Select
               tstRange.Select
               ActiveWindow.Zoom = True
               .Range("a1").Select
           End If
       End If
       End With

    Next wks
    Sheets("cover").Select
Application.ScreenUpdating = True
Application.Calculation = xlCalculationAutomatic
End Sub