You want your pumpkins to be three different sizes: a small, a medium, and a large. You could even go further if you want to, but it gets hard to find pumpkins that big. ;)
Roughly measure the pumpkins in relation to the other ones. Set them up. Does is look nice? Can the biggest one eat the medium one, and can the medium one eat the small one? If so, you've got your pumpkins.