I haven't ridden a Big Dummy, but I had the same experience with the Xtracycle Edgerunner. I'm 5'7', so comfortably in the S/M size range and at the bottom of the L and the S/M felt ridiculously small, not just the top tube length, but also the seat height. I had to put the seatpost past the minimum insertion to feel comfortable riding but the L felt just fine.


You might contact Madi Carlson, whose blog is at: https://familyride.us/ about what size she rides.