Originally written and owned by Carol Spears
The first thing that you might notice is that GIMP opens the image at a logical size for viewing. So, if your image is really big (like the sample image) it will display it zoomed out until it fits nicely. You can tell if GIMP has done this by the percentage number in the title bar. (you can click on the little screenshot to see the full view of the screenshot) Just because it looks right in this “View” doesn’t mean anything.
The other thing to look at in the titlebar is the mode. If it says RGB in the title bar, you are fine. If it says Indexed or grayscale there, you should read the Change the Mode Quickie.
Image entry in the menu and the sub menu from the screenshot should reveal itself. Click on “Scale Image…”. When ever you click an option from the menu that has … behind it, expect another dialog. This time, you should get the “Scale Image Dialog”.
If you have a desired width, put it in the dialog at the top where it says “New Width”. If you don’t have such a number in mind, you can steal the width of GIMP’s default image size, which is 256 pixels. This is demonstrated in this screenshot. You can see the image that this scale dialog produced here.
Perhaps you want your image to look more like a 4×6 inch photo on most image rendering web browsers. Simply switch the units to “inches” and put 4 inches in the height box (opting for smaller than 4×6 rather than bigger). You can see this dialog here. The image this dialog produced here.
Let GIMP choose the other dimension length for you. Meaning, it will take more image knowledge to change both width and height and have it look correct. So only change one and let GIMP change the rest. To change the other length see the crop quickie.