The right way to do this is something called Responsive Design. Rather than try to predict what screen sizes there are going to be, make your app adjust automatically to wherever it is run.
We have an article on Responsive Design in the Wiki - it may be a good place to start.
I read that article - twice - but still couldn’t get it to work. Ended up with lots of controls with negative numbers for top and left, etc. I wonder if you could post a sample project that uses responsive design, so we can see what has to be changed from the default settings. I gave up and used absolute as well.
Responsive Rotation: Shows how to handle portrait and landscape, as well as positioning buttons differently based on the width of the screen.
Grid: Shows how to use Bootstrap Grid elements to position different areas of the form.
I would be misleading you if I said that Responsive Design is as easy to do as Fixed. It isn’t. There are a whole bunch of new complications and considerations which apply.
I generally place most controls and inputs within a container (left: 0px; width 100%, height 100%) and controls/inputs ect set them to left 5%, width 30% etc…
I don’t think the actual form size matters as it will always fill the entire screen (Project Properties > viewportFit: cover). I only change the form size if i need to when designing the layout.