You can't really do AM3+ at anything below ATX.
ITX boards basically don't exist at all (though there have been a few exceptions in the past).
mATX boards are hugely limited in selection, to 760G chipsets which are fairly antiquated.
If you want smaller motherboard AMD builds, you are stuck with FM2/FM2+ boards, though depending on your price range and usage demands, you might well be better off looking at the Intel alternatives, as they are much better supported across the range.