If you're wanting to breed easterns I would go with at least 2 years old for both the male and the female. Size depends on species and really is no indication of sexual maturity. I would say if your female is at least 18 inches (24 would be better) long and 2 years old, she can be bred. If you wait until she's bigger you could have better luck with offspring numbers and size though.

As was said before, the male can produce offspring at a very young age, and sometimes without brumating him, but you should choose a male that is about the same age as the female, or younger, and definitely choose a male(s) that is a little bit smaller than her.