If your solution is for a specific company - ask the IT staff or interview people that will be using it. Find out what users are using.
If it is for a general audience, you can specifiy the minimum screen resolution that your software requires. Design your interface - that should be an indicator of your window size. Just becuase you may have the screen real estate does not mean you have to use it. A window that is too large is actually a hinderence to productivity as people have to scan left and right too much. FileMaker layouts can be semi-responsive to differnt window sizes and grow with the available canvas, but is is far from perfect. Alternatively, create layouts for different screen sizes and go to them dynamically based on the user's screen size. This option is a pain for management and changes however.
There are also sites where you can research trends in screen resolution to see where your sweet spot may be.
On Windows you also have to determine whether FileMaker is maximized or your database window is maximed within a non-maximized application window. Window management on Windows can be a bit of black art. OS X is easier in this respect.
A system-critical solution does not have to use the entire display.
Bottom line - your FileMaker solution should drive the interface not the other way around. Function over form. Usability is key.
Thanks for your thought! Dave!
The maximise setting will adjust the layout to what ever screen it opens on, regardless of design. If the designed layout is too big for the screen the RHS and/or bottom of the layout will still be off screen. I suggest you take the approach of finding out what will be the smallest screen in common use and design your layouts to fit that. Then for those lucky people with larger monitors the maximise setting will enlarge your layouts to full screen size, and you can use the autosizing inspector panel to determine how the layout will change when enlarged.