Well i'll just tell you from personal expierience, as long as your hatchling is a good eater, you really wont have to worry about the food size, since msot of the time if you can't get a rat pup, you can always get an adult mouse. Or if you need a Rat pinkie, just go for a smaller adult mouse, about the same size.

I just sold a male Het Piebald that was 200 grams december of 2009 and was 1000 plus grams when i sold him a month ago in October, he just shot up in size and was my best eater.
I fed him every 4-7 days depending on avaialbility of rats, so he took them very easily.

You can feed every 4 days if you want, but not a real big difference between 4 and 5 days, so just more money and food over time.