Right now this bimac has a mantle that is about the size of 2 to 3 golf balls stacked together. It might reach a mantle size of about 5 golf balls, which is the largest one I've had for a pet.
As for the spike effect, I'm not quite sure what causes the horns. Obviously, octopuses have awesome control of their coloration, due to their ability to quickly change their pigmentation. And I think they also possess the ability to change their cellular texture and cause stacked skin invaginations to produce the horns. I've noticed that they appear when they octopus is excited at feeding time. As soon as it sees me coming up to the tank with a crawdad, for example, its "horns" pop out above its eyes and in different locations on its mantle.