I have a 30lb piece in my 60 gallon. After mixing in some nice live rock pieces and some corals, a few months later you can't even tell it's limestone. It's heavy and pretty dense, but it's got tons of caves. Every inch is covered in something live.
While we're on the subject, Home Depot used to sell (don't think they do anymore) Pavestone brand crushed limestone. $5 for 50lbs... heck of a bargain considering 20lbs of "live sand" costs ~$20. Used 4 bags on my 140 gallon tank and it is awesome. Makes a cloudy mess of the water for a few days, but if you're cycling your tank, it doesn't really matter.