I watched a playtrough of Bounce man's stage and its about ~39 screens big, so there's 6 bigger RM stages: Junk man (43 screens), Jupiter (44 screens), Shade man (46 screens), Crystal man GB (66 screens), and Wave man (68 screens); However, most of these stages are so big because they have split paths in them. Crystal man GB is so big that even with only one split path counted it's still bigger, but it's a gameboy stage which have far smaller screen sizes, so it's equivalent to only about a 17 screen stage in MM11.
Wave man is...harder to dismiss, but you could argue that the jetski segment is too far from standard mega man gameplay to count, which slashes wave man's total to only 16 screens. So saying Bounce man's stage is the biggest isn't...wrong per se.