Here are some rocks that I cleared from my tank that are for sale. Total is about 40kgs. Asking for $5/kg. take all for $160. The size of the tiles are 30cm x 30cm(1 ft x 1 ft) So you have an idea of the rock size. Unfortunately the pics can only show 2D so you cant see how exactly it is like but it gives an idea of the size.
Here's a link to the pic : https://lh3.googleusercontent.com/-wZeS82wW9UU/UPPblJqf8kI/AAAAAAAAA7g/AdLOMxqxOmk/s640/IMG_0218.JPG