The size of the rock varies as well as the shape. A lot depends on how dense the piece is. When I ordered 100 punds I got 102 spread out over 18 pieces. I have one piece that is about the size of a football and very light. Another piece is somewhat smaller but weighs a LOT more because of the density. As a rough guess I would say 15 pounds is about the size of 3 footballs, more or less. You can ask for sizes and/or shapes and they do try to accomodate, but their stock is in large boxes and they have to pick from what they have accessable. If you can ask for 1 large rock, you can break/cut it to suit your needs.
If you have a specific question or need a more accurate estimate you can write to them from their web page and they will get an answer back to you in short order, usually the next business day at the latest. These guys have always been quick to answer and offer good advice.