Add a little bit of salt (not too much) to the water and collect the spray on a transparent plate. Measure the contact angle of the water with the transparent plate and determine the size of the droplets on the plate with microscopy. All salt will be distributed to the edge of the drop, so you should be able to see the drop radius. There is a mathematical relation between the radius of the drop on the plate and the radius of the spherical droplet in air. r/R=(4(sin(th))^3/((1-cos(th))^2*(2+cos(th))))^(1/3), where r = radius of drop on the substrate and R the radius of the sphere.