In my house I usually play 2''-2,25'' chessboards.
Outside home, I take with me sets that fit well in 2''- 2,25'' or 1,85'' - 2,00''. Sometimes I carry 1 rollable chessboard of each size.
I have found that many sets are really playable with both, 2'' (50mm) and 2,25'' (55mm), those are, so far the best sets for me.
For me, the "magic point" is: pawns with a base of 1,10''(28mm) and kings with a base of 1,46''(37mm).
With this size, it is usual to find available chessboards wherever I go, so, most times I don't use mine, I just take the pieces out of the bag and play.
Also, as I'm used to this size I feel unconfortable with smaller sizes.
In bigger ones, I need to get more focused to avoid missing anything.
Anyway, with near 50 sets at home and a strong addiction to chess sets it is kind of epic to choose a chess set when I go out.
You know, the set that's not too big, or too small, for over the board play.
Given enough table space, I prefer a 4.25-4.5" king on a 2.5" chess board. Anything larger and I start knocking things over reaching for the back ranks.
As the playing area gets smaller, I scale down my sets to fit the available space. With enough extra room for elbows, captured pieces, a scorepad, chess clock, and my favorite beverage or brew.