Males usually can be smaller than females but I wouldn't try less than 1.5 years (IE to breed in '08 you want an '06 or older)
I don't know about grams - I think a well fed corn 30 inches long should be fine - whatever it translates to into grams. Properly fed for the length I think is more important, a 300g snake can be underfed and possibly not advisable to breed if it is long.
My '06 hatched in August and is already about 27 inches.
Also - I think some localities (IE Miami) are naturally smaller and breed smaller.
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6.8 L. getula californiae - 11 eggs (Cal. King)
1.2 L. getula nigrita (MBK)
1.0 L. getula floridana (Brooksi)
1.0 Pantherophis guttatus guttatus (Corn)
0.1 Pituophis catenifer catenifer (Pacific gopher)
3.3 Elgaria multicarinata multicarinata - 14 eggs (Cal. Alligator Lizard)