I have cropped a bunch of comps using the region of interest tool and now I want to increase the size of the region of interest for those comps or simply reset it to the original comp size... how do you do this?
Long and short: You can't. It's one of those dangerous things in AE - everything is relative to the comp and when you mess with that it's a one-way trip because there is not even a genuine abstract 3D space or an overarching infinite 2D canvas. you can try to find soem comp resizing script on AEScripts.com or use the default comp resizer that comes with AE, but more or less you may be screwed. Not all properties and keyframes respond correctly to these upsizing operations and you will have to redo at least some stuff.
Cropping a comp to the region of interest resets the comp center to the center of the region of interest. If you then go to the Composition Settings and reset the comp size you have options to size from the corners, edges, or the center. None of those will put the comp center back in the same spot that it was unless you are extremely lucky. All resizing a nested comp is going to do is change the position of that comp in the main comp. If you need it bigger, collapse transformations and scale it or make the layers 3D and move the camera.
The Option is to resize the main comp, then collapse transformations on all of the smaller comps and all of the layers in those comps. There is a chance that this will foul up some of your comps, but collapse transformations will allow the main comp and/or the main comp camera and lights to use all layers in the nested comps even if the layers are outside of the nested comp's layer boundaries.
Cropping a comp to the region of interest should be the first thing you do before you do any animation in that comp. Work that way, keep all of the animations inside the comp frame, and you can use True Comp resizer from aeScripts.com to resize very complex projects without problems.