The type generally found in live rock and coming in with corals is the Pacific Bristleworm, which can grow quite large, but which is a very essential assist for the corals. It digests fish and snail poo into a size of particle useable by sps coral. Some experts argue that without them in the tank, sps wouldn't do as well.
I've also seen scary photographs of tank bottoms where no worms exist, and poo just lies there a quarter inch deep.
They eat only already-dead things, and do not attack, though I've seen one wrestle with a hermit or a plate coral for a fish-pellet...the worm can win against the coral, but not the hermit. They will sting your hand painfully if you touch their bristles, but can crawl across a coral without damaging its delicate skin. And they clean holes in rock that nothing else can reach, thus preventing nitrate buildup.
There is no such thing as a 'dangerous size' imho. I have a couple of 10 inchers and one nearly that big, and they're mild-mannered and generally appear only for food they've learned they can eat...won't stir for cyclopeeze, but pellet food will bring them in a hurry.