Why font size does not (really) relate to how big it is
[…] I set out to find out which part of my 15px Verdana is actually 15px in width or height. The only problem was that I could’t find any part because there was none.
According to Thomas Phinney’s great Article em equals the font’s point size which is a grid of an arbitrary number of units (mostly 1000 or 2048 UPM/Units Per Em) into which the glyphs are drawn.