Snapping with 'region of interest'

I'm trying to pre-compose a shape, and to have the dimensions of the comp match those of the shape layer:

This seems surprisingly difficult... first I was looking for a way to simply 'trim' the comp size like I can in Photoshop, but this doesn't appear to be possible.

Then I found out about the 'region of interest' tool, but there doesn't seem to be a way to snap it to my shape layer. I can't even snap it to any guides that I put in place.

Is there a way to use this tool more accurately or do I just have to zoom in to 1600% and do everything manually?