Shame it needs a bloated SDK package else I would have given it a go, but if this is upscaling does it then mean I am encoding a higher resolution video meaning a much bigger video?
This is the same SDK package required for other nVidia video effects that OBS implements itself directly, such as background removal/blur. This SDK does not run in the background, or continuously consume resources while installed, it's only requirement is some hard drive space, but I do not work for nVidia and that's as far as my trying to defend anything about this goes.
This plugin adds a new filter you can add to certain sources, such as video capture device sources, that allows the source it's attached to to have it's image processed by the nvidia super resolution video effects, and output that source at a higher quality scaled size than what OBS natively implements with it's scaling (bilinear/bicubic/lanczos) .
It does not necessarily mean a bigger video as this only effects a source in your scene, unless you decide to increase the canvas size you're rendering at in OBS itself to accommodate the increased maximum size of the source itself. This is up to you.
For example, I normally stream/record at 1080p or 1440p, and use this plugin to scale lower resolution video captures (480p/720p) up, and then scale down to my canvas size.
Note that this plugin does NOT magically make the source appear to be rendered at a higher resolution, it just has the ability to make sources look far less blurry when scaling them up than the built in options OBS provides.