Go to:

C:\Users\YourNameHere\Documents\My Games\Borderlands 3\Saved\Config\WindowsNoEditor

Open the file "GameUserSettings.ini" with any writing tool.

Find the line: EdgeDetectionTexelOffset=-1.000000

That's the outline thickness. On default its set to -1.000000. I like my outlines a bit thicker, but not too thick.

• If you are playing in1080p, change the value to 2.500000.
  
• If you're playing on 2K, set it to 5.000000
  
• Disable outlines by changing the value to 0.000000

The higher the value, the bigger the outlines.

(ALSO WORKS FOR TINY TINAS WONDERLANDS)

I originally posted this guide on reddit 3 years ago but I updated it with the 2K res number today and suddenly the post was "removed by reddits filters" and I lost my profile picture there. Unfortunate but whatever. So it's here now lol