If you create a new image with the size of two by two inches and don't have right-click View Dot for dot enabled, then you should be able to the measure the image directly on the screen to a size of two by two inches.
If the size you read is wrong then you have to adjust your monitor resolution in this dialog.
The GIMP is using a manual setting by default (72dpix72dpi). If you run GIMP on a Unix workstation then you can probably use the "From windowing system" 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 windowing system. You have to measure and set those values by hand which can be a bit bulky. There are two fields — one with the default measure unit DPI and one with custom measure unit which can be useful for those who are used to the metric system.
You don't really want to calculate your resolution manually, so simply press the Calibrate button to pop up a dialog where you can measure two rulers and enter their lengths in spin buttons.