My one or two attempts at birch went south very early in the game, for reasons like too many projects and not enough time.
There are a few birches posted in ''Other Deciduous", not a lot. The best looking ones the owners did not try to keep them too small. Most were somewhere in the 2 foot to 4 foot tall range. 2 to 3 feet seemed most common. Most of the smaller trees did not seem to survive the 5 years in a bonsai pot mark. Just an observation, I'm not saying it is impossible to do a shohin birch, I just haven't seen one that turned up in photos more than once.
Others with more experience should add their tips.