The type size determines how large the type appears in the image.
The default unit of measurement for type is points. One PostScript point is equal to 1/72 of an inch in a 72‑ppi image; however, you can switch between using the PostScript and traditional definitions of point size. You can change the default unit of measurement for type in the Units & Rulers area of the Preferences dialog box.
To use an alternate unit of measurement, enter the unit (in, cm, mm, pt, px, or pica) after the value in the Size text box.