Pretty easy questions so far . . . well at least wait until sexual maturity, which the general rule of thumb is two years. Size is important as well, so around three feet in length would be sufficient. Breeding too young/small can be problematic, particularly around egg retention. When breeding kings, ALWAYS watch for signs of aggression and potential cannibalism. Sometimes you may see a male king grab a female with his mouth behind or even on her head. That's normal behavior, but if it becomes too aggressive and constriction ensues, time to intervene. You will also see a form of foreplay which involves the male twitching his body while he tries to align with the female. This is normal as well. I had a great pair of nigrita (Mexican black kingsnakes) about 10 years ago in which the male would immediately go for her neck, bite, and completely overwhelm the female into submission. They would be in full copulation in under 20 seconds. So sometimes it may look aggressive, but it's really about dominance. Keep us posted when the time comes.
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Jerry Kruse 
"One often meets his destiny on the path he takes to avoid it". - Master Oogway