This information is presented for the sake of completion and historical accuracy. Don't use the <BASEFONT ...> tag. It will make your life difficult. See Break the <FONT> Habit for an easy tutorial on styles. If you would like to learn how to set the font for the entire page see Setting the Font of the Whole Page.
SIZE sets the default font size for the page. The size values for SIZE works just like those for <FONT SIZE="...">.