Try 1366x768 as game resolution. This will probably produce the same ingame object size as 1024x768, but is a 16:9 resolution and you will not have black bars on Youtube. You can upscale this to 1920x1080 with OBS, so you will not lose any picture information. On the other hand, this will result in blurry edges and increase bandwidth requirements, so downscaling to 1280x720 is an alternative to that. Downscaling to that wil not bloat the video and not result in increased bandwidth requirements.
You can also use exactly 1280x720 as game resolution and stream this resolution. This way you get exactly the video on Youtube you experience ingame. This is better than playing in 1366x768 and downscaling. Upscaling this to 1920x1080 makes no sense, since upscaling will not add any picture information - only make images blurry and bloat bandwidth requirements.
Streaming or recording in 1920x1080 doesn't make sense as long as you don't play the game with this resolution. For recording, record exactly the resolution you are playing with. No scaling at all will always produce the best picture quality.
