I put a bean screen in a 7700 several yrs ago , I had the same problem , plugged all the time and was running excessive return , don't know the exact size but it was JD oem bean screen round holes supposed to be just what I needed .I took it out after a few days put the old sieve back opened it all the way and controlled cleaning with the top chaffer , worked okay . I do the same for corn also . JIM