It depends on what shape you consider a raindrop to be. If we are thinking of a spherical shape, only about a 5mm diameter. A water molecule is pulled equally in all directions by the neighbouring molecules. As a result of this, water squeezes up until it has the lowest possible surface area. The shape with the lowest possible area is a sphere, therefore a drop is formed. Surface tension forms and this becomes increasingly unstable as the size of the sphere increases. The surface tension can no longer hold the spherical shape so the shape of the sphere is distorted, no longer being a raindrop (assuming a raindrop has a spherical shape).