I\'m trying to do what the title says. I\'ve seen that font-size can be a percentage. So my guess was that font-size: 100%; would do it, but no.
font-size
font-size: 100%;
Can't be done with CSS.
100% is in relation to the computed font-size of the parent element.
reference: http://www.w3.org/TR/CSS2/fonts.html#font-size-props
For a jQuery solution look at Auto-size dynamic text to fill fixed size container