If you create a image new image with the size of two times two inches, and don't have right-click|View|Dot for dot enabled. Then you should be able to take a folding rule, and the measure the image directly on the screen reading a size of two by two inches.

If you can't i.e the size you read is wrong then you have to adjust it in this dialog. Gimp is using a manual settings by default (72dpix72dpi). If you run Gimp on e.g a Unix workstation then you can probably use the From X server option and get a correct value. If you run on PC hardware you will most likely not be able to use the value given by the Xserver. You have to measure and set hose values by hand which can be a bit bulky. There is two fields one with the default measure unit DPI, and one with custom measure unit which can be useful for us how are used to the metric system.

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