They are about the size of your pinky nail. I had a pair in one of my big acros that I lost when I had to treat for red bugs. They do need either an acropora or a pocillopora to survive, and I doubt they would do any harm to any of your fish. They actually will never leave the coral, and will hide in its branches should anything go after it. And yes, completely reef safe, considering it never leaves the coral it hosts. Very cool little animals should you have the right coral for them.
And on a side note, the coral can't be a little frag either, it needs to be a decent sized colony, at least the size of your palm I'd say, with many branches.