If the end users are using plugin client. then you can provide a button on the page to say "fit zoom to window" and call a macro like below.
SUB FitZoon2Window
ActiveDocument.ActiveSheet.FitZoomToWindow
ActiveDocument.GetApplication.WaitforIdle
ActiveDocument.ActiveSheet.ApplyZoomToAllSheets
END SUB
Else, if your users are on the ajax client. then, my fried you suggest them to use the zoom options on the browsers itself to adjust the zoom. Or you can hold the control key and use the scroll on the mouse to adjust the zoom.