You can find all the info you need on the Cub Cadet website. Here is a link to your deck, specifically for the PTO to deck belt. If you click the Add to Cart button for a belt, it'll usually give you the size on the pop-up window.
http://www.cubcadet.com/equipment/AR...0-A/0074200001
To find parts for your tractor, go to their website, click the link to Service & Parts on the right side, and type in your model number.