I did not try it in word as I cannot adjust the pic in gimp and transfer it. I did take margins off and I am using legal sized paper.
If you are still resisting Photoshop then
USE WORD!!! ha ha........forget this gimp thing, surely you can just save the image to your desktop as a JPEG or PNG and then insert/import into any other programme you want?? and then you can adjust the size of it perfectly easy!! I do it literally every time I print as I I like my UGC covers a tad smaller than the standard templates/covers are saved at.
Man, I know I/many people here could fix this for you in about 8 seconds if we were there.

Here, in Word, go =
1 - Set page to landscape
2 - Go to Page Layout
3 - Select Margins - then custom margins - change them all to 0.5
4 - Now got to Insert - Picture
5 - Once imported, right click on it and select "Size and position"
6 - Look to the bottom of these options and un-tick "Lock aspect ratio"
7 - Re-size the image now
8 - PRINT!!!!

..........but make sure, if you are printing on A4/legal paper, that you have chosen the right print option as there is one for each.