[untamed]

As a family of worms, the Eunicid (Oregon) are pretty common. Different species acheive different maximum sizes though. I've kept a number of them in my tank for years. At one point, there was one living in the sand bed that was over 2 feet long. (He stretched nearly 3/4 away around the circumference of the tank)

I've observed the largest to eat mostly macroalgae. The smaller ones seemed to be scavengers of whatever they can grab. I long suspected that they are the reason that I can't keep Xenia in my tank, but have never actually caught one in the act.

As for bristleworms, it is pretty easy to separate the good from the problematic. There are some that will eat coral, but in order to do so they need to latch onto the coral and scrape at it for hours. As such, it is pretty easy to catch them in the act and remove them with tweezers. If you just see the worm in/on the sand, I would leave it be - even if it seems large. (unless you just don't like seeing them at all, but that is an entirely different concern)