You are getting spotty information because females can (and will) breed at various lengths and weights. While there are some people who love to spout the "3 years old, 3 feet long and 300 gram" phrase, it's useful to keep in mind that some female (as well as male) corns NEVER reach 3 feet in length. This does not mean they shouldn't be bred - in nature Miami phase and Keys Corns routinuely breed at lengths of less than three feet.
As far as weight, you should be able to look at your snake and know if it's too skinny or not. Also, snakes that are too fat will sometimes have trouble laying eggs (presumably because they do not have enough muscle tone).
I realize that newcomers want a "cook book" recipe on exact weights and exact measurements. But the truth of the matter is that snakes are animals, and therefore subject to a great deal of variation. Not only that, but Corn Snakes in particular have a very wide range with sub-tropical examples (south Florida) and temperate examples (New Jersey). There are Okeetee races which generally are considered to be larger than average and Florida Keys races of Corn Snakes which generally are smaller than average. There is a lot of variation in Corn Snakes.
This is why Corn Snakes can breed at various sizes, ages, brumation temperatures, etc. This is why there is no "right" answer to your question. As a keeper of snakes it's up to you to be able to judge if the snake is in proper condition to breed. If you feel you cannot do this, I'd have an experienced breeder check the snakes out and tell you what to look for.
I've been breeding corn snakes for over 20 years and have produced thousands of them - I have yet to weigh one.
Tim

Third Eye