Back to the "How Small is Too Small" topic: My observances and research have led me to believe age/size does matter but not in the production of large clutches.
Observe the young BP, observe the size of the eggs that must pass though the vent. The size of the eggs does not get smaller in relation to the size of the BP laying them; ergo, common sense leads me to believe that using smaller bps whose vents may not stretch enough to pass eggs successfully without stress to her, may not be a good idea. Using a younger BP who may not have enough fat reserviors in her and good muscle tone for breeding may not produce successfully laid clutches. Those using smaller/younger BPs and announcing success, may not be sharing all the information. Some of us have used children (12) year olds for example...yes, it can be done, and successfully with a doctors help, the gravid 12 year old doesn't have the formation, or girth to be able to successfully give birth to a 6-9 lb baby.
My research has taken me to other forums where I have read threads of spontaneous abortions in BPs and one theory was that the snake was not large enough to carry all the follicles that had been fertilized in her.
My continued research has given me an understanding that while size should not matter in terms of mating, it does matter in terms of size of eggs, ability to carry the eggs to term and lay them successfully.
Regarding the common belief that BPs bred young will lay smaller clutches in the future, there is no scientific basis for this belief. Most breeders use the "hit or miss" method of breeding. Research has shown me and verifiable data is available out there for those that research, that it's not size alone that gives a snake the ability or inability to produce large clutches, it's the NUMBER of follicles that are the correct size to be fertilized when the fertilization happens. If only a small number are the correct size, then the clutch will be small; however, if there happens to be a larger number of follicles of the correct size available for fertilization then the snake will lay a large clutch. This is why some breeders, who have researched, have begun using Ultrasounds to track the growth of their BP's follicles.
Put together, the animal must have the size to be able to carry the eggs successfully and pass them without losing too much weight or becoming egg-bound (which can also happen in a snake with some age/size, but other issues may be at play in those cases).
The above statements are simply the collection of information that my research has garnered and my own observations of my snakes and their formation.