Remuxing isn't able to change any properties of the recording such as resolution. It just changes the container format, for example *.mkv to *.mp4. What you want to do is achieved by recoding. This is extracting the unencoded video data, perform the transformation you want to do (resize to a different resolution), then re-encoding again.
Handbrake and ffmpeg are tools for this. OBS Studio has no function for recoding, it is only able to remux.