Tzeentch laughs at likely
Join Date: Dec 2009
Location: Inside my Head, Bristol, UK
From memory, the Army Book does not specify a size so it is whatever seems right.
From a pure fluff (so not rules binding) perspective, it is an item for a character and is called a shard, so I see it as being small enough for easy transport: thus it might work well as approximately Beastman sized on a 20/25mm base; however, a large rock mounted on wheels on a chariot base is also reasonable.