I don't use Yast so I have to ask the question, can you set the font size in that application separately?
In lxde I believe that the fonts are stored in /usr/share/fonts. (Not sure if that helps either.)
There are a couple of posters that use Opensuse with lxde that check in here at the lxde forums Perhaps they will have a better solution for you. (Solution? Sorry, I did not provide that.)
Lubuntu 12.04, Lubuntu 12.10, Lubuntu 13.04, and Lubuntu 13.10 on an ASUS EeePC 1000H