maximumSize
Gets or sets the maximum size of the cumulus cloud rendered on the billboard. This defines a maximum ellipsoid volume that the cloud can appear in. Rather than guaranteeing a specific size, this specifies a boundary for the cloud to appear in, and changing it can affect the shape of the cloud. Changing the z-value of maximumSize has the most dramatic effect on the cloud's appearance because it changes the depth of the cloud, and thus the positions at which the cloud-shaping texture is sampled.
To modify the billboard's actual size, modify the cloud's scale property.