This Gui Ying Stone from Aqua One is completely natural, and therefore every single piece will vary. We're unable to confirm sizes or shapes, as these are based on weights, and every piece will be unique.
Available in a 20kg Box, or individual pieces of around 15 - 30cm each, these are a grey stone, often known as Grey Pillar Rock. These are a limestone based stone, so can cause some hardness within your water, especially if you're using a CO2 system, but as long as you're performing regular water changes this shouldn't have much affect.
We have a great range of decorative rock available, which you can find here.