My "S" layout is very small, but many of the buildings are "HO" Woodland Scenics Ready-Build structures. Some HO looks too small, some just right. My "test" is to view it with S automobiles and people figures. If they look OK, I'm good to go.
I started noticing that different brands of HO buildings seemed to be different sizes. I also asked members of a huge S modular layout at a show what they used. They pointed out a number of HO buildings that sported S size doors.