I'm trying to think of the last time any of the people I played with played anything smaller than a dwarf which was towards the top of the height range for a dwarf.
I had a sidekick who was a Ghostwise Halfling at one point. That's literally the only size S 5E character I've actually seen at a table I was playing at. Wow. That's kind of surprising. Even the guy who always players "joke" or "weird" characters has never played anything less than size M in 5E.
In earlier editions we saw a few. I've often had people threaten to play a Halfing or Kobold, but then you see their actual PC and it's something disappointingly normal.
So this isn't something I've had to deal with for a long time. Previous to now I found the best way was to have some stuff they just have a much easier time fitting through, and some stuff they're going to need a boost up and so on.