Sub MyZoomIn()
   Dim ZP As Integer
   ZP = Int(ActiveWindow.Zoom * 1.1)
   If ZP > 400 Then ZP = 400
   ActiveWindow.Zoom = ZP
End Sub

http://excel.tips.net/Pages/T002731_Zooming_With_the_Keyboard.html

Notice that the macro only allows you to zoom in up to 400%. This is because Excel allows you to only zoom that high, and any higher would generate an error. A slight variation on the same theme results in a macro I call MyZoomOut. It zooms out of (reduces) a worksheet by 10%:

ZoomIn Excel Macro

Visual Basic