I always prefer to go by weight. Do you have a kitchen gram scale you could use to weigh him? You can also eyeball the size - a good feeder size should be about 1 to 1.5 times as wide as his belly. Personally, I prefer to skip double pinkies and go to peach fuzzies when they're around 15 grams. Pinkies don't have much nutrition and two of my pinkies end up being larger than one peach fuzzie.