As far as I know, there is not official GW stance on base sizes other than the generally accepted standard of "use the bases that come with the figures."
On a related note, I sent a couple of emails off to the rules email after the Skaven book came out, since there are a number of options that don't have official figures, so we're left without base sizes for our converted & scratch built figures. I've received no answers, though in that time other people have had their emails answered. I can only assume they are avoiding answering the question.