You can add product upgrade pricing by going to Admin > Products > Website Products. At the bottom of the page you can change the name of the product upgrade (eg. Two Dozen, X-Large) and add a price.
Note: Please be sure to add only the incremental amount, not the new total price.
You may also default upgrade names so that all upgrade options will have the same label by default. This option can be found under Settings > Store Info > Website Product Settings. These labels may be overwritten on individual product pages as needed.