yep, common bristle worms don't hurt anything that is healthy. they just eat uneaten food, fish poop, and dead things. if you have large ones, it's probably because you over feed a bit.
^ I've seen this video, either in the description or in the comments he states that he found out that it was a fire worm, not a bristleworm. BIG difference.
As far as size, I'm pretty sure I've seen a thread somewhere on the net of a guy with a worm like 5' in length, I don't remember if that's a bristleworm or not. In my 14G I had one that was easily 1ft+ when stretched out, needless to say I removed him
they are relatives of the earthworm. i have seen them the size of those large night crawlers. the more food is made available, the larger the population will be. if you saw one, you have more. no worries though.