If you are running KDE4.x Plasma, either search in Synaptic for kde-gtk-config, and install it, or open a terminal and:
sudo apt-get install kde-gtk-config
System Settings > Application Appearance.
In the left pane choose GTK.
Among other things you can adjust the font.
Mine was some weird Ubutu font with a size of 14. Changing it to the font and same size I use in my KDE-qt Programs worked perfectly and seems to stick across things like Synaptic, Chrome, GIMP and Geeqie among others.
If you modified things according to my previous post you might also have to go to Fonts while you are in System Settings and drop Force Fonts DPI back to 96 or turn it off.