From Online Manual
description: The size tag formats text between the BBC tags to be displayed at a specific font size. The recommended size formats use the points, pixels, and relative-size keywords, as defined in the CSS2 specs. This is an inline tag and no block level tags can be placed inside, however other inline tags can be used if nested correctly.
example: [size=x-large]text or more BB Code[/size]
result: text or more BB Code