Is the layout zoomed?
Is this on a windows system where the display control panel is set to a larger value than 100%?
Is "Enlarge Window Contents to improve readability" specified in Preferences? (Windows only).
What size do you get when you print and measure the printed output?
Stephen, the size on the screen is never going to match a real life size, unless by accident. A simple example:
Put two monitors next to each other, e.g 24" and 27" with the same resolution. Now if you display the same window with the same settings on each screen and measure it, the size will be different.
Measuring the screen with a ruler to get the size is never going to work. I think cm will only work if you print your layout.
OK. So I should just trust the size guidelines (eg. for iPhone) in layout mode when deciding how big to make my layouts?
I would strongly recommend that you use FM GO to open the file as a client via WiFi while you are developing the database on your computer. While the stencils are accurate, they aren't complete--don't allow for every possible layout design and their can be small differences in how layout text, etc appears on your iOS device when compared to what you designed in FileMaker, so this arrangement allows for quick review and adjustment.