Individually they are small but they form colonies & those can get huge. We've had some as big as a small beach balls but most are basketball, watermelon size. Yeah, these things are nasty looking. They are pretty stinky too when you smash them up as you remove them from the objects they are attached to. Apparently you can use a liquid copper sulfate solution to control them, but I haven't looked into the cost. I know copper sulfate can be a problem long term as it settles into the muck/bottom.