I’ve used 20mm since I first started with Airfix plastics back in the 1970s 🙂 It’s still my preferred “larger” scale, although I’ve picked up a few 28mm sci-fi and fantasy figures.
20mm seems to be reasonably well represented, although it does feel like 28mm gets the most online love.
I used to have some 6mm, and still have a bit, but for smaller figures, I prefer 3mm nowadays. 3mm seems to be a very niche scale, and as such there doesn’t seem to be a lot of online coverage. That doesn’t really bother me, though, to be honest.