The largest gammarus I've ever caught is 1/4" (we're talking about wild ones here... I occasionally go down to Carmel (near Monterey CA) and look for sticklebacks & stuff in the Carmel river. Anyhow, the largest ones I've seen are about 1/4", and most aren't any bigger than 1/8".
I'm not sure your fish will eat them though. They have very, very, VERY tough shells, and from what I've read, not much other than adults of the various big cichlids (Oscars, Discus, Angels, etc.) have jaws strong enough to crunch them.
However, like you said, they're probably not doing any harm. They can carry diseases, but if they've been in there for years and you haven't had a problem, I doubt you're going to have one now. Besides, they're scavengers, and in the wild at least, they burrow into the sand a short ways, so they're probably at least somewhat beneficial.
I wouldn't worry about it.