You can add this code as a macro and it will zoom to window:
Set s = ActiveDocument.ActiveSheet
'Apply zoom to fit window for current sheet
'apply zoom to all other sheets
There is no possibility to zoom in the AccessPoint, so I think you're talking about Desktop or Plugin users.
Write a macro like presented in the first post of this discussion: Fit zoom to window Macro in Access Point
Attach the macro to a button using an action type External->Run Macro
Note that you cannot let a desktop window resize trigger this macro automatically, because QlikView has no events for this.