Locate the model number of your John Deere riding lawnmower. The model number will be on an identification label, with the John Deere logo and the serial number, on the mower itself, as well as in the user's manual.
Open your Web browser and navigate to the official John Deere website.
Type the model number of your John Deere riding mower into the "Search" box then click on it. This will pull up the product page for your riding mower. The product page has various technical specifications about the mower, including the deck size.