Age is more important with bloods, than size. Females need to be at least three years old to breed, and often we'll wait until a female is 3.5 or 4 years old before we put her into our lineup. We consistently have better first-year results with girls who are given the extra time to grow up a bit more.
We've had males that were ready to go quite young by comparison, but really prefer them to be at least 2.5 or 3 years old - again, much better results with giving them the extra time.
Our breeder adults are in the 4' - 5.5'+ range, length-wise, and range between 12 - 15 lbs. Some of our biggest girls are over 6' and weigh 20 lbs or so.
HTH!
K~