reduce font size below 6 in emacs on SHR

[robin]

does anyone know how I can reduce the font size in emacs even lower than 6?
6 is the lowest number I can choose if I use the GUI to change font/fontsize.
As the terminal can produce quality small fonts I would guess that it is 
possible in emacs as well. Maybe it has something to do with the SHR scaling
factor set to 2. Though that is needed to have the icons being displayed in the
right size...

best regards

robin