Application brief SLMA004B states, "Solder mask should be applied to cover the entire bottom side of the PowerPAD vias to prevent solder wicking. This includes full solder mask coverage over the vias.

WSON's need to have appropriately sized pads for proper fillets and to prevent wicking, otherwise the joints may be unreliable. If you do have to make some strange sized pads, I would double-check ...