Advanced Configuration Editor

First open the Preferences by hitting Ctrl + P, then click on the Advanced Tab as pointed below:

http://img261.imageshack.us/img261/2610/p19la.png

then click the Open button under Advanced Configuration Editor:

http://img157.imageshack.us/img157/5793/p27yt.png

then you can change a lot of options of Gajim. Here I searched and I'm about to change the size of avatar in roster:

http://img157.imageshack.us/img157/7229/ace1il.png

You have to double-click in the "Value" column of the line to modify it. If it is a boolean (True or False), it will change it to the other value, if it's another type you will have to modify the value with keyboard.

NOTE: Changing values via ACE can make your Gajim unusable. If such a thing happens, you can remove or edit your config in ~/.gajim/config but in case of removing it that means you have to start from scratch (add accounts etc..). You will not lose your logs.