While the width of the letters increases as the font is sized up, the width of the spaces between each letter decreases. At the exact size of body text, called typical body text (TBT), intended for general newspaper use, most commercially available fonts have x-heights of just over a third of the body text width (MOE (typeface margins of error) as detailed below). So at 12 point type, a space that's 5.5 points in height uses ~2.75 em.
The original study reported by various newspapers (typically in the ITU font standards book) was done on the Ideal Mono printing plate. Often, in the age of two-color process type when newspapers were published (1950 to 1993), a third size was added (called "3S" or Standard) which was half the point size. Since that period, the 3S, or Standard size, has been removed from the systems for it was very prone to mechanical printing errors, so it's actually a pretty good estimate of typographic norms. The change in bias was thought to make letterforms more readable, and thus to improve advertising effectiveness. Before “typography” became a discipline, printers believed that, rather than type size, the point size was the most important hint.
The optical size is the size of the type in the printed image viewed at an angle with normal paper. In the United States web standard ISO 9550-1 the measurement (in dots) of the letter size is referred to as the point size. Whitespace includes spaces and any special characters (such as bullets or underlines), and is always explicitly stated as part of the whitespace font size. d2c66b5586