GIMP crop does not support simple selection of standard print sizes, this liquid rescale tool could do the same by liquid expand or shrink to the correct sizes selectable from radio buttons (e.g. 6x4, 7x5, 10x8 etc.) These would need to be in the select new width and height area of plugin dialog - in reality it is controlling the aspect ratio but users can understand the output sizes easier. "Expand to" and "Shrink to" would have to be an option to and possibly the dpi?
Standard photo print size support