I have many images with about 20 products per page that I need to crop into single images.

They are the same item but different sizes, so I need the images to have exactly the same relative size to each other. If I use the crop tool in photoshop it just limits the aspect of the select box to the aspect entered, even when they are pixels, so if I put in 300px and 300px and drag a box, and then drag another box around another it will just resize the box to be 300px.

Is there a tool that will give me a fixed size crop frame, basically when the images are displayed beside eachother the relative size of the products in shot needs to be preserved, which isnt something I can do by eye and I would rather avoid resampling if possible since the images from the supplier are already barely at screen resolution.