My client game size is set to 1152 x 648 (16:9 aspect ratio) this makes the scaling fit perfectly into a 1920 x 1080 output canvas in OBS
Depending on your computer setup some of these settings in OBS may be to taxing on your computer and may have to lower the quality if you are having issues - you can start with the preset
Edit: I forgot to mention I also use stretched mode which is built into runelite, if you want to copy my setting for that just below
Increased Performance mode: Check
Integer Scaling: Uncheck
Keep Aspect Ratio: Check
Resizable Scaling: 13%
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