When you look at the EH extracts from Magic map, they tend to give the impression that it's the whole boulder/outcrop, at least the ones I've seen for Northumberland. There's often mention of the possiblity of undiscovered carvings nearby, so if interpreted in a hopeful light, this could be taken to mean that the scheduling might cover adjacent outcrops too.
But I wouldn't be surprised of the extent of the area under protection differs from site to site.