The size of characters in a font is usually measured in points, where a printer's point is 1/72 of an inch. So the tall characters in a 12pt font are around 1/6 of an inch high, whereas a 14pt font is about 1/5 of an inch and 7pt font is about 1/10 of an inch. However, the exact size does vary between fonts. On screen the physical size of pixels (the screen resolution) varies. Old displays often used to have around 70 pixels per inch, so a point and a pixel were nearly the same, but now that varies a lot more, particularly for phone displays and retina displays. The device tries to choose a pixel-height that closely matches the paper dimensions of the desired font. If the precise screen size of characters matters, the font size may be measured in pixels.
