This is not something Zoom can do natively - you can remove the 'HD' option in Settings->Video to crop from a 16:9 to a 4:3 ratio, but this is detrimental to image quality.
If your webcam's manufacturer provides a utility, you may be able to change FOV there (Logitech does this on certain models, for instance).
I'd also recommend putting in a feature request (I already have, but numbers help) for "Near-End Camera Control" - this would allow you to zoom in on just the area you wish to show via native controls baked into Zoom. Right now, Zoom only allows far-end camera control - very odd that you can't control your own camera but others can control it for you.