According to Archaeology, Constantine's giant finger (from his colossal statue) has been found in France. As it turns out, the piece has been in the Louvre since the 1860's when it was misclassified as a toe. So, it's true that Constantine was guilty of giving the finger to his various foes and in some cases, family members that threatened his grip on power during his long reign. No pictures of the finger in question have been released as of yet (sigh!) but here is the head of the colossal statue along with the Great's hand.
I'm not sure @Sallent - we'll have to wait until some pictures appear. When available online I will post an example.