I don't do flats, so can't speak to that. I do use imagine bamboo prefolds stuffed in BG 4.0 As a reference, DD is on the medium rise and the medium prefold is a little big, but not too bad.
Proud mommy to PJ, born 6/23/2012
and CR, born 8/11/2014
, wife to CE, since 9/20/08