how do i crop a photo for a specific exported dimension?

Firest crop the photo in iPhoto to the size ratio of the final exported size.

Next, during import use the custom size options and enter the maximum dimension of any side of the image.

That will get you the exact dimensions you're looking for.

This user tip explains the options in the Export panel, including the ones that allow you to export to particular dimensions.

https://discussions.apple.com/docs/DOC-4921

You can download Snapseed

Snapseed on the App Store

it will allow you to crop many dimensions such as 3:2, 5:4, 7:5, etc.