Most of my favorites are 22-24 mm badgers!! These are just great for face-lathering!!
I prefer brushes with short loft. The Semogue 610/620 are two of my favorite brushes, so I would be tempted to say that I like smaller knots (21mm). However, my absolute favorite brush is the Omega 011842 and its knot diameter is 27mm. The knot size is small though since my copy has a 48mm loft based on my measurements.
@Sotiris_A: When you use your (21mm x 50mm) 620 do you find that you need to keep going back to your puck for more soap - assuming you face lather?