Getting current Frame's changing Image size


I’m looking to get the current frame of a read nodes actual image size, so that I can scale and position it to the final resolution.
I normally render lower resolution test or previs and then want to see the final output format and scale the images to fit so as the final renders are generated at full resolution the rest of the pipeline is in the correct format.
It seems the normal way of getting the transforms input’s resolution is a ROD that is not aware of changing frame sizes.