Font size specifies the display size of text characters. In CSS, the font-size property accepts various units: px (pixels), em (relative to parent), rem (relative to root), and vw (relative to viewport width).

In web design, 16px (1rem) is the standard base for body text, with headings scaled up according to hierarchy. CSS design fundamentals books teach font sizing principles.

Responsive design increasingly uses the clamp() function for fluid typography, specifying minimum, preferred, and maximum values so font size scales smoothly with screen size.

For accessibility, rem units are preferred over px so users can adjust font size through browser settings. Responsive web design books cover practical font sizing strategies.