Hi, Martin. You're not the first to ask.
From the site:
2 Responses to “2015 Sweet Corn Prizes in Fiction and Poetry: Now Open”
24 Mar 2015 at 5:21 am
Do you accept previously published work?
25 Mar 2015 at 4:43 pm
Blog Editor says:
Thanks for your question. We do not. Keep writing and send us your fresh work!
Line limit: I'm not aware of any & think you should use your judgment. If I hear anything different, I'll post it here. Thanks!