Currently the only way to SPECIFY a cropped page size in Acrobat is to use the clunky preflight tool. Why not simply allow the "Change Page Size" settings in "Set Page Boxes" dialog to result in a smaller page size (crop).

If you try this now, you get a "Page size may not be reduced." error. In the crop dialog. The place where you go to make the page smaller. The actual definition of cropping. Why not just let it crop to the size you type in, whether that's larger or smaller?