Font size is measured from small to large through a point size system that is universally recognized by all computers and word processing programs. According to Computer Hope, "A font is typically measured in a point (pt) size, which is the vertical measurement of the lettering. There are approximately 72 (72.272) points in one inch or 2.54 cm."
Font size is also measured according to em-space and ex-space standards. Em-space refers to the width of the uppercase letter "M" and ex-space refers to the height of the lowercase letter "x". These two comparisons set the standard measurements for all alphanumeric values, allowing font size to be displayed consistently. The em-space and ex-space concepts helps all sized fonts to become standardized and consistent throughout web design programming and is used in HTML code.
Font size can also be measured in pixels. According to About.com, "This refers to the number of pixels in the viewing device. Font sizes are determined relative to the viewing device resolution." For example, a smartphone screen translates font size into smaller pixels than a standard laptop computer screen. The physical size of the screens are different, even though the web page information is consistent throughout multiple devices.