The characterists of boar are so different you may not like a small knot in it. For example, the Omega 31064, with a 26mm knot, will look and feel and work like a knot thats smaller than a 20mm badger knot. Badger hair blooms, boar hair clumps together when wet, its very different.
I have the Omega 10029 and find it too small, the 10048 is OK but a little too long of loft for a lot of guys. The 31064 is the perfect size for a boar imo. It also looks very nice.
Let's start with the universal B&B disclaimer; YMMV!!
Having that said I am going towards ever smaller boar brushes; most of all I am picking brushes with shorter hairs. Started out with around 60 mm. there was an Omega travel brush that I liked so much that I wanted to use it daily but the used materials are not up to that.
So my quest ended (for the time being) at this Vulfix boar and I love it.
But if you are still a boar maiden you better go for a larger brush to start with; break it in properly (around 15-30 shaves) and then decide what to go for!
As others have noted, Omega mades some smaller brushes. I highly recommend the Vulfix 2233 boar brush. This is one of the new premium boar brushes from Vulfix that have lathe-turned handles. I believe it's around 17mm knot size. Overall it is about the same size as my Savile Row 3118 (18mm knot). Looks and feels compact, but the handle gives it a very nice heft.
The bristles are not quite as long as Omega brushes, and the head is not quite as dense, though it has plenty of "backbone." The tips are surprisingly soft right out of the box and get softer after a month of use. Mine cost $22 including shipping.