If you use the trigger OnFirstWindowOpen and in that window run a script that creates a new record to a Log table you can see what your users are using in window size
Get ( WindowWidth ) & " - " & Get ( WindowDesktopWidth ) & " - " & Get ( WindowContentWidth ) & " - " & Get ( ScreenWidth )
Then you can use the Logs to check to make sure you build a layout based on what your users are using.
If you want to modify the size of the application window inside which all other FileMaker windows open, there is no such function. You would have to run a system script or something.
Thanks Johan for the info but, my intention is to define in advance the size of the application window.
I suspected it...