Não é uma boa notícia para você, mas do fórum Arch Linux 1 parece que o gtk dev decidiu para retirá-lo e não substituí-lo.
GTK+ 3.10 intentionally doesn't pay attention to "gtk-icon-sizes" because the current developers want it to have less "obscure" options like this. I have talked with them about it on IRC and unfortunately they aren't likely to change their minds.