There isn't a single specific name for a large boulder. You could call it:

* A large boulder (simple and accurate)

* A giant boulder (emphasizes size)

* A massive boulder (emphasizes size and weight)

* A rock formation (if it's a notable geological feature)

* A glacial erratic (if it was transported by a glacier)

* A monolith (if it's tall and stands alone)

The best term depends on the context and the specific features of the boulder you're describing.