Here it is, step by step.
First, open a whole lot of jpegs into ACR (as many as your computer can handle, this varies from computer to computer. Hit Cmd A to select all of them:
Then go to the Crop Tool, and choose 5x7 in the drop-down. Immediately this crops ALL of the images to 5x7:
Then quickly click on them one by one to check that you're happy with the crops, and modify if you're not:
It's SO much faster than the old way of manually dragging the crop marquee for each photo.