You the crop tool as an adjustment, set the aspect ratio to 3:2, start at the top left corner of an image, and drag to the bottom right and make it as large as you can. Once you’ve made it as large as you can, if it doesn’t touch all sides, move the image around in the crop box to make sure you include all the pieces you want (if possible).

When you export it, it will produce a 3:2 aspect ratio output file.

Exactly how many pixels you need depends on the size of print and the pixel density. For example, a 4x6 inch print with 300 pixel density require 4x300 = 1200 and 6x300 = 1800 for height and width. A large 20x30 would be 20x300=6000 and 30x300=9000 pixels.

You can do the output sizing yourself, or you can let your print house do it for you. If you let the print house do it for you, just output your image at the “original” size. That provides them all the pixels you have. They can take it from there.