On Tue, 14 Sep 2010, Henning Hraban Ramm wrote:

Ever wondered about what exact size is some relative "name" like \tfa? I did some calculations for you, enjoy: http://wiki.contextgarden.net/Font_Sizes

Please anyone explain where we need "script/small/big" sizes.

script is needed in math. Big and small provide a complete font change. http://wiki.contextgarden.net/Font_Switching#Complete_font_change Aditya