Just set up an Xubuntu system with a few KDE apps, all of which had minuscule menu fonts. The simple solution is described there and my version of ~/.kde/share/config/kdeglobals looks like this:
[General] desktopFont=Bitstream Vera Sans,12,-1,5,50,0,0,0,0,0 fixed=Bitstream Vera Sans Mono,10,-1,5,50,0,0,0,0,0 font=Bitstream Vera Sans,12,-1,5,50,0,0,0,0,0 menuFont=Bitstream Vera Sans,12,-1,5,50,0,0,0,0,0 shadeSortColumn=true smallestReadableFont=Bitstream Vera Sans,8,-1,5,50,0,0,0,0,0 taskbarFont=Bitstream Vera Sans,10,-1,5,50,0,0,0,0,0 toolBarFont=Bitstream Vera Sans,10,-1,5,50,0,0,0,0,0 widgetStyle=qtcurve
And then It Just Works… the alternative being to drag in the whole KDE configuration tweakage infrastructure, which I’m trying to avoid.