That's easy to answer - the very tall and very short creatures. The average height of most aliens we encounter are roughly around caucasian adult human sizes, so if you're, say, a Gabilani or Raskvel, you're quickly too short for things; same goes for aliens much bigger than average, either in length (like naga), mass and leggedness (like taurs) or height (anyone else who's "too big").
At least one or two scenes mention as much, at least for taurs (I think Ushamee and Aina have text regarding that particular problem), spaces need to be made taur-friendly (wide entrances, taur-friendly spots on public transport, which there are never enough of, particularly long household items like broomsticks etc.). In the end, it really means a lot of "irregular" people will have to compromise since I doubt most public spaces go out of their way to provide much in terms of appropriate accommodation for outliers because it tends to mean extra expenses for custom objects (like chairs that can tank a taur and not crumble like a pitiful house of cards). What's more likely is that there are establishments that cater specifically to such a clientele (very huge and roomy for unusual body types and heights above 8ft, small and cozy ones run by aliens who're short themselves, like Raskvel, Gabilani and Toves...)
What I'm saying is, you don't have to do much mindbending magic to figure out how this can reasonably work.