I'm a fan of the Wee end of the spectrum. In fact I tend to consider anything over 20 or 21 mm to be wasteful. Biggest brush I actually use is a Semogue 620 I got in a PIF, which I would happily buy a 20 mm or smaller version of and re-PIF the 620.
Somehow over the years I have gravitated to larger and softer brushes. For many years, my favorite (and only) brush was a Simpsons Duke 2 in Best Badger. Now my favorites are a Maggard 24mm SHD Two Band and a Yaqi Moka Express 26 mm Two Band. While the Maggard is only a 24mm knot, its bloom is quite large. I also have some 22mm brushes and some smallish 24mm brushes that I use from time to time.
I mostly use Yaqi brush brand and they seem to have it figured out for my shaving needs.
24-26 in synthetic brushes are very nice and of lately prefer the the 26mm finding them not lather hogs at all.
22-24 for badger brushes is usually a nice spot for those knots.
19-24mm for boar brushes works great for my shaving needs.