I voted 22mm, but if it's not at the extremes it's fine with me. I can't imagine a really small brush holding enough lather. Big brushes just feel sloppy.
For me, it's a little less about knot size, and is how densley packed the brush is. That said, I have an old Rooney with a knot of 26, and it is packed so tight i'm a little afraid some morning while using it, it might expload!
I prefer a smaller knot. I picked 20mm. It gives plenty of coverage and the pack is nice and firm. Floppy brushes bug me and they cover too much area for my liking.
