The crop tool has that option. As far as I remember (not running windows atm) just click the drop down arrow and choose a pre-defined ratio or simply enter your own in the boxes.
That sounds right.
Can the size be adjusted but it will still stay in it's set ratio? If so that's perfect
Dont forget though when you change the aspect of a picture ie process a portrait instead of a landscape you will need to either rotate the image or swap the numbers round.
Or much easier click the little arrow between the numbers to swap them around for you.
