Using GIMP: Correcting and Touching Up Your Images - GIMP 2.6 for Photographers: Image Editing with Open Source Software - page 92

Cropping the Image by Numeric Input Using
the Image > Canvas Size Option
You already set the output size using Image > Scale Image . In this case, there is
a second possibility, cropping the image numerically. This can be done using
the function Image > Canvas Size .
Figure 2.27
The Image menu
The Set Image Canvas Size window appears (figure 2.28) . Image editing
programs call the workspace where the image is displayed the canvas . The
canvas can be larger than the visible image content but by default, the canvas
size is the same as the image size.
If the canvas is enlarged, an additional image area is displayed around the
image. This can be used for adding further image elements or text. However,
the actual image remains the same size. See section 3.13.2 for an example of
enlarging the canvas to add additional images.