the following are gide lines of the span in metres the rib and block slab can span length wise.with the normal house loads .
150mm slab ( 4m - 5m )
200mm slab ( 5.6m - 6.6m )
250mm slab ( 7.3m - 8.3m )
please consult you structural engineer , about the span and load ratio, the load imposed on the slab will determine the span and the amount of reinforcing in the slab to counter the load.
rib and block is a one direction slab , meaning it bends in one way.