Check out the Method Shaving section of this site. I think there are a few posts where different brushes are mentioned. I personally have used the 30mm Vulfix #41 and have found it to work very well. I would guess that the 20mm brush you have might not hold enough water or "mix" to get through an entire method shave without having to remake the lather. This is, however, just a guess. Give it a try, and if it doesn't work well, look to upgrade.
...the police often question him, just because they find him interesting...his blood smells like cologne...