In an architecture office, the term "subway tile" is mostly used to describe a standard, white, rectangular tile that's 6" (wide) x 3" (high). As I take a closer look at the subway stations in the city, I realized the 4 3/8" x 4 3/8" square tile is just as, if not more common, than the 6 by 3 one. They look like this:

Here is their scale in relation to a person:

6" x 3" rectangular vs. 4 3/8" square subway tile

And in context:

6" x 3" subway tile in Columbus Circle Station

4 3/8" square subway tile in Times Square Station