You mean how do they compare at the same overall bow length? Different riser lengths with the same limb lengths will yield different overall bow lengths.
For the same length bow, say 60 inches, a longer riser, say 18 inches as opposed to 16 inches, means that the bow's limbs will be shorter, and shorter working limbs are supposed to be a little quicker/higher performance. Longer limbs are supposed to be a little more stable, and perhaps draw a little more smoothly. However, the differences are slight, and I doubt that most shooters would notice them. A larger riser adds more mass to the bow, of course. Some folks like that.
I agree with Curtis that for any given bowyer, a given riser length usually works best with a given limb length. Ask the bowyer for his opinion of the best combination at each length.