My wholesaler sells petals in a white garbage bag. He told me that the bag equals about 75 rose heads.  I measured last time, and it equaled 72 cups of petals.  For me, that would do one side of a 10-row aisle with medium coverage nicely.  I measured using my 8-cup pyrex measure (9 of those full).  Seems like a gallon container (I'm thinking paint can) is pretty close to the same size.  So to do the same, for both sides, would be 18 gallon-containers full for 10 rows.  So round up to 2 gallon containers per row for both sides, times 7 rows, maybe, 14 gallon-containers full?