Changing Image Width and Height
Your GIMP program will now open up with your image in the editing window. Go to the Image dropdown menu, and select Scale Image from the list of available options. This opens up the Scale Image dialog box. In this box, you’ll see selections for image width and height.
You can resize your image based on items like pixels, percentages, inches, points, picas and millimeters. One of the easiest ways to resize your image is via percentages, which is the second option in the dropdown box next to Width and Height.
For example, if you know that you only want to make your image 20 percent smaller, you can use the down arrow to 80 percent in the height and width. When you change the dimensions of the photograph, you’ll see the resulting pixels underneath it. This is important if you have a pixel limit on a photograph, which is common on social networking sites.
Unless you want your image slightly distorted, always leave the aspect resolution locked. If not, then your image will not be proportional.