IMPORTANT! ALWAYS BACK UP GAME FILES BEFORE EDITING THEM!
Now, I am not exactly sure on the repercussions of changing it to an unsupported resolution, but I can tell you how. Navigate to your
\Riot Games\League of Legends\game\DATA\CFG
By default this will be in your Local Disc drive (C:\)
If you are prompted by a message saying it is unable to locate the program to open it with, just choose the select from list option, and find notepad.
Once you have opened the Game.cfg file in notepad, you will see:
Change this to your desired width and desired height and save the file. So for you it would be:
Hope this helped, have any problems or concerns, just post them here or email firstname.lastname@example.org
... I'll try to help ya out.
Edit: Also, remember that standard monitors are 4:3, meaning that the width is 4 times the height. And wide screen is standard at 16:9. ( I believe... ), Just use one of the standard resolutions, like 640x480, 800x600.... or anything that is standard for widescreen; or else your screen may seem a little stretched