I haven't seen a snake that was healthy weigh more than 300g and be less than 3 ft, so I don't think you have to worry about the length. And I don't think age is that important either (assuming they weren't power fed); most will be breedable their 2nd year. For me the most important factor is that they are nice and lean (not fat!) and are close to if not over 300g.

If I ever have any doubt about breeding a female because of her size, I always wait until later in the season if not next year; I never push it.