Ok, I think I know what you mean. If I bring in an image to use with the compositor, say it’s 1024×1024 pixels, but the Blender render settings are set to 1280×720, the 1024×1024 image will not automatically fit the render settings dimensions. In that way I’ve dealt with confusing dimensional discrepancies within the compositor.
Are you using an external image file with the compositor that doesn’t fit the project’s dimensions?