The rock with these measurements are not uniform at all - it's the size of screen used to sort it. Often times, larger pieces will get thru with all the bouncing (or just bounce over the screen into the pile). When using media from the landscaping stores, I use 3/4" lava rock - lava is really light and pourus, but it's sharp, so if you're planning on using worms, you'll have to find another place for them in your system. I saw one guy who had his return line pour into a pail of filter floss, and put his worms in there.