fluff wise, as far as they need to be :)
You'll find the actual ranges in metres in the RPG books: A Time of War has them in the gear section. The brackets are Short / Medium / Long / Extreme.SRM (Standard) - 60/225/425/740SRM (Light) - 55/180/270/475SRM (Heavy) - 74/235/353/530 LRM - 75/400/1000/2100