It is a myth and one that has been around for a long time.

When I see snakes 'sizing up prey', what they do is look at it. As in, sit there for a moment, I can see their eyes moving as they look up and down and then they decide it's too large and don't strike. I have NEVER seen a snake strike, coil and then do anything to size it up and have never seen one lay alongside something before eating it. They decide before striking if they can handle it, at least in my experience.