The first thing that comes to mind is to use a Browser Source. It's a pared-down version of Chromiuim, so it may or may not include that functionality. If it does, then it works just like a regular web browser except you can size and position it as you need.
If it doesn't do that, then you can use a full browser (pick your favorite) outside of OBS, and arrange its window to make things a bit easier in the next step: Then Window Capture it in OBS, and crop/size/position from there.