If the drip isn't from a thread or seal, then there could be something obstructing the nozzle, might be worth carefully sticking a fine pin in it (not too roughly, presuming it's a brass nozzle, otherwise you'll mangle it up!).
You'd probably get away with going down to an 04 or even 03 like Len, I suppose it might depend on the model of sprayer and the pressure it's being used at. If you put too small a jet on it, you'll get back-pressure - what's coming from your pump will end up going the wrong way down the venturi pipe and fill up your sprayer's container.