"To help customers, SmugMug's shopping cart only shows a print size if the original image meets our minimum resolution requirements for that size. Keep in mind that these are minimums. Please read "Recommended settings", below."
"In your Customize Gallery settings?, you can choose the largest size you'd like available. Standard and Power users can choose to display images only as big as SmugMug's XLarge size. Professional subscribers can choose to display images only as big as SmugMug's medium size."
Medium:
pixels wide - Up to 600
pixels high - Up to 450