When the riser cards are seated in the slots on the motherboard they extend 6in (15cm) above the surface of the motherboard.
FWIW, the motherboard and risers fit nicely in a CoolerMaster Stacker case with an additional inch to spare.
From front to back, they are 6.5in (16.5cm) long.
The remaining dimension is that both risers mounted together (in their purpose built mounting cage supplied with the mobo) add up to 4in (10cm) wide. This dimension includes the protrusion of installed sticks of memory.