To resize, which is probably what you actually want:
Go to image, resize/resample. Uncheck "preserve aspect ratio". For "set new size", select "inches" and then enter 7 in both boxes. The image will be shrunk to 7x7.
To actually crop it to 7x7 (you probably don't actually want to do this):
Go to edit, create custom crop selection. Set width and height to 7 inches. You will need to change the units to inches. Press ok. A selection box will appear on the image. Put your mouse cursor in it and hold down the right mouse button. Move the cursor around to move the box. When you have to to where you want to crop it, go to image, crop selection. Everything outside of the box will be eliminated.
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