1. The best way for (b) is to export in a highly compressed LOSSY format. I recommend exporting the image as a .JPG at 80%-90% quality (using the slider). You can aim for a specific file size (b) by ticking the preview box in the gimp export dialogue. Export as an 80% jp, then as a 90% jpg (with diff name - doh), then open both the jpgs side by side and see if you can tell the difference. Often you can reduce as image size (b) by about a tenth or more with no visible loss of quality in this way. Note that because your jpg is lossy you should not depend on it for long-term storage as it will degrade when manipulated (&saved) so make sure to keep your original gimp files safe and make new .jpgs as required rather than amending your existing jpgs.
2. The other way is to scale the image in terms of pixel size, reducing the amount of pixels across and tall. You of course want to make sure to keep them in proportion. Reducing the pixel size (a) will inevitably lead to a reduction in file size (b).
Hope that helps.