Yep they come in a handful of sizes. The more common size for these protests seem to be around 2 inches in diameter.
To put that in perspective, try to make the largest circle you can make with your index and thumb (think OK symbol shape). That's more or less the shape of these things and they are effectively a thin layer of rubber with a solid steel core. The inside of your finger circle is the steel and the thickness of your fingers is the rubber.
It does help, thanks, and it's also terrible, because it sounds like the police would think "oh it's fine, these things aren't lethal" and then shoot people willy-nilly with what are basically big steel cylinders.
To put that in perspective, try to make the largest circle you can make with your index and thumb (think OK symbol shape). That's more or less the shape of these things and they are effectively a thin layer of rubber with a solid steel core. The inside of your finger circle is the steel and the thickness of your fingers is the rubber.
Hope this helps.