Dang... I wish I had taken a picture yesterday of mine. I have 5 on one frame. They look like acorns - about as big - on the top half of the frame. I dunno, probably protruded 3/8 " off the frame.
I scraped one and it definitely had a larva in it. Some bees build them just to tear them down, or have a couple around for emergencies. Mine are, more than likely, superceding cells. The girls must not be happy with their queen for some reason.
Hope that helps!