(updated once I parsed the wiki)
There are character profile cards coming out with the DVDs in Japan that apparently have all the usual data (ht/wt/bday/etc). If we know their birthdays and assuming they're born in the same year (highly likely in Japan's system).
Found the japanese wiki for K-On!
Yui: ht 156cm, wt 50kg bloodtype O bday: November 27
Tsumugi: ht 157cm, wt 53kg, bloodtype O, bday: July 2
Ritsu: ht 154cm, wt 48kg, bloodtype B, bday: August 21
Mio: ht 160cm, wt 54kg, bloodtype A, January 15
Source - japanese wiki for K-On!
http://ja.wikipedia.org/wiki/けいおん!
So, Mio is oldest and Yui is youngest if they're all the same year birth, imagine that....
Also, all pretty average height and weight for japanese late teens.
PS: Azusa is ht 150cm, wt 46kg, bloodtype AB, Nov 11
(hmmm, same height and weight as my wife ... o.O)