Since we're not even seeing him, it's awfully hard to say. You don't want to see a big bump after a meal, just a slight one. If you're not sure, what I'd do is alternate sizes for a while- one meal, the smaller size, & the next, a slight bump up in size, so as not to overwhelm his digestion too quickly. It never hurts to continue with smaller ones, but you could also just feed a day or so sooner as another way to compromise. Just remember, nature isn't perfect either- snakes can handle some variations- perfection is not essential.