This may be basic, but I can't figure it out.

The browser/layout page size defaults to 21cm wide in accord with my printer settings (A4 size), which is about 600 px.

I want to set it up for typical screen use which means 800 px would be reasonable.

I don't understand why I can't just grab the edge of the page in layout and drag it to the size I want. Is that too much too ask?

It would seem so, as the only way i can find to do this is to change the print setup to landscape format instead of portrait.

This is an unsatisfactory workaround, as it relies on the printer to set the screen size.

Is there a better, simple way to set screen display size to 800px wide? Or better still, by checking the computer's display setup and formatting the layout to suit the computer user. (as can be done with web pages by getting the browser's setting)

Bruce -- don't confuse the page boundary with screen size. They are not at all related.

The page boundary only comes into play when the layout is previewed or printed, which should be never if it is going to be used for data entry.

It's best if you use separate layouts for data entry and separate layouts for printing, as generally a layout optimised for printing is not optimimal as a data entry layout.

If it's a data entry layout you can ignore the page boundary and design to whatever size you wish.

If it's a print layout then you must ensure that you keep everything to the left of the page boundary, because noting on the right hand side of the boundary ever gets printed.

If you want to know what the computer's screen resolution is, look into the Get() functions. FMP 7 introduced a lot of screen and window related functions for controlling window.