Your font size number ranges from 1 (very small) to 7 (very big). But it depends on the type of font your using.
To change the size of font you need to put:
You can also add other things to change the look on your font. Here's a list of some of them:
font-family:HERE; - changes the type of font. (arial, georgia, tahoma, etc.)
color:#; - change the fonts color