Well, that's going to depend. The water that is coming out of your sprinkler is coming out a small hole in the sprinkler. Drops of water in the air are roughly spherical due to the surface tension of water, so the water droplet will be a sphere with a diameter equal to the diameter of the whole.
The volume of a sphere is:
(4/3) * pi * r3
where r is the radius (not diameter) of the sphere.