This DIY project requires a few special tools, but the good news is that they're cheap and oh-so-worth-it if you're desperate for a just-right length of ethernet cord, Goldilocks.
Sick of too-long and too-short cords? Take matters into your own hands (and your own ethernet crimpers) and make your own ethernet cord.
The instructions from MakeUseOf give you a pretty detailed run-down of how to do it (plus pictures), but you'll need a few special tools first: An Ethernet crimping tool ($10-$20) and an Ethernet connector (the little plastic end piece; 50 for $9).
Once you've got your tools, head over to MakeUseOf to follow the detailed instructions on how to shorten that cord for good! (This is also a helpful 'how to' if your Ethernet cable, especially the little plastic ends, ever break. Just cut off the wire and re-crimp another connector on.)