Have you set any of your layout objects to resize, stretch, or "float" unanchored? This would allow items to reposition based on the size of the current window.
You will also need a script which adjusts the size of the window either to maximize the window, or to reset the window size based on info from the Get functions for Desktop sizes.
Otherwise, windows will open in the size you last left them when developing in single-user mode offline from the host.
I should of thought to maximise the widow. That worked well. It must take the window size of when it is running in Pro by subtract the FileMaker toolbars and Windows Toolbar.