If you intend to open the image in Gimp again for further work, you should save it in Gimp's native XCF format (i. e., name it something.xcf), because this is the only format that guarantees that none of the information in the image is lost.

If you intend to print the image on paper, you should avoid shrinking the image, except by cropping it. The reason is that printers are capable of achieving much higher dot resolutions than video monitors---600 to 1400 dots per inch for typical printers, as compared to 72 to 100 dots per inch for monitors. A 3000 x 5000 image looks huge on a monitor, but it only comes to about 5 inches by 8 inches on paper at 600 dpi. There is usually no good reason to expand the image either: you can't increase the true resolution that way, and it can always be scaled up at the time it is printed. As for the file format, it will usually be fine to use JPEG at a quality level of 75 to 85. In rare cases, where there are large swaths of nearly uniform color, you may need to set the quality level even higher or use a lossless format such as TIFF instead.

If you intend to display the image on screen or project it with a video projector, bear in mind that the highest screen resolution for most commonly available systems is 1600 x 1200, so there is nothing to gain by keeping the image larger than that. For this purpose, the JPEG format is almost always a good choice.

If you want to put the image on a web page or send it by email, it is a good idea to make every effort to keep the file size as small as possible. First, scale the image down to the smallest size that makes it possible to see the relevant details (bear in mind that other people may be using different sized monitors and/or different monitor resolution settings). Second, save the image as a JPEG file. In the JPEG save dialog, check the option to “Preview in image window”, and then adjust the Quality slider to the lowest level that gives you acceptable image quality. (You will see in the image the effects of each change.) Make sure that the image is zoomed at 1:1 while you do this, so you are not misled by the effects of zooming.