If you're going to breed a male, you probably want to breed him when he's 3 feet long or so, and atleast a year old. That having been said, this year I bred both a 3 foot hypo and a 32" (at the time) anery to my female, and only the anery sired any offspring.
As far as males go, if they're roughly 3 feet and if they seem genuinly interested in the female, go for it. Breed multiple males as some might not be very fertile.
For females, no shorter than 36", and you do want them to have good weight and muscletone, else you run a high risk of egg-binding and/or nutritional deficiencies. NEVER breed a sick or anorexic snake.