AE has no auto-crop feature. At best you coulkd set a render region in the comp viewr and then use Composition --> Crop to rimited region. Other than that of course it may be a good idea to just let AE already determine the comp size on import by choosing a specific layer in a PSD or AI file and setting the import to layer size...
I think if you're coming from photoshop (where artboards can easily go unnoticed) confusion for me in this area is between the relationship of a layer and composition. Is a composition not more like an art board? I feel like it regardless, neither of them have the properties people are looking for when considering to use them. Or at least I just learned about this 'crop composition to area of interest' tool, yet it still is not what I'm looking for. Take a photo for example in photoshop. You can toss a new layer in that's larger than the canvas. Until you crop it or change dimensions of the canvas, the overflow of the layer goes hidden but stay is still there if you slide the picture around. It's only after sizing either through cropping or editing width and length values of the canvas that, once set, will result in the layer itself being cropped. I want this same precrop feeling. I need to make sure the overflow, if possible, stays in existence because I am only trying to show the region WITHIN an 'area of interest.' I am trying to convert 180 video into something watchable on YouTube. To do so, I need to retain the effect of the vr converter tool on the video layer. But not have the composition size determine where or how big this layer is. When I focus on one area, what I hope would happen would be that the effect continues acting on the whole size of the layer, but that the only seeable area, is like a 2:1 rectangle much smaller than the layer. Because after cropping, the effect readjusts to as if the layer is the new crop size.
Would the solution be to create the size of a composition first. Then add a video to that comp and add an effect to that layer? I guess I have always been trying to crop comps... which makes it seem more like how photoshop works but the confusing part is, even after a comp is cropped, it still shows an outline of the bounding box around the video... as if it still exists in that size. !!!! Or Is the solution to create a comp from the video file, add the vr effect (assuming they act on comps, then add that comp to a comp that's the size of the window you want to show? Then not use a be converter effect on this view area comp? Maybe that would focus the vr effect on the comp you'd like and not the one you need to proportion to an ear you want seen?!?
There are standard frame sizes and frame rates for video that must be followed if you want the public to see your work. You would never want a composition video that you were trying to share with the world to fall outside of those standards. That does not mean you cannot letterbox an HD project to give it a 2:35 Cinemascope look, it just means that you don't want your comp to be a non-standard frame size.
You can, and it is often a good idea to create smaller or even larger comps that you will nest in the final standard frame size project, but the most important part of video production is designing your assets - the images and graphics that you use in the movie, so that they are at or close to 100% scale when they are in their hero (most important) position in the frame.
Please explain the need to create custom comp frame sizes a little better. I don't think it is a good idea to fit the comp size to the artwork without considering the size of the final project.
If you already have an asset, an image or a graphic, or even a video, and you want to create a comp that is the same size as the asset all you have to do is select the asset in the Project Panel and choose Create New Comp from Selected in the File menu or drag it to the Create New Comp icon at the bottom left of the Project Panel.