Forcing Fullscreen at a custom resolution

Solution 1:

From what little research I did, it seems that any methods of changing graphics quality and setting custom resolutions within the UE4 config files are outdated and won't work anymore, but there is a 'proper' way of setting custom resolutions.

You can create custom resolutions in your AMD drivers, allowing you to select them in the game, if the game supports custom resolutions, rather than only allowing pre-selected resolutions.
(These instructions only work for AMD gpu's)

1. First you need to open your AMD Radeon Settings by right clicking on the desktop and selecting the AMD Radeon Settings option
   
2. Click on the Display tab on the bottom of the settings window and click + Create where it says Custom Resolutions
   
3. Change your Horizontal resolution and Vertical resolution to your preferred values and click ✓Save
   
4. After you have saved your custom resolution, it should be available to pick in the game.

There is a chance that Fortnite might not pull the available resolutions from Windows, but rather have a list of pre-set resolutions instead, in which case this will not work.