I used this snippet in the past with success,
<!-- Scaling a chosen font with Fontconfig. By poige, 2008. --> <match target="font"> <test name="family"> <string>Liberation Sans</string> </test> <edit name="pixelsize" mode="assign"> <times><name>pixelsize</name>, <double>1.1</double></times> </edit> </match>
Here is the source of this snippet, but in Russian language.