I have a Rab Neutrino Endurance down jacket which I use in winter Snowdon/Scotland during the freeze your bollocks off weather. It is size medium.
Now I want to buy a Rab Latok Jacket as a full weather/waterproof but ideally it would be nice if it fits over my Down jacket as well just myself.
The question is, what size should I buy if I want something that will fit over my down jacket for when the temps drop and I'm stuck up on Tryfn or the Cairngorms belaying for an hour. Likewise if it's not cold and I just need to wear the jacket on top of a fleece or something? I fit size medium well with room to move, though I fit snugly in size small as well though not a comfortably.
Would I want a size medium Rab Latok jacket and just squeeze into it wearing my Down or opt for a large with a bit more room when wearing the Down and very roomy without the down?
If it were me I'd being going for a waterproof that fits, in your case a medium, and wearing the down jacket over the top as a belay jacket. If its too wet for a down jacket, then it probably isn't cold enough really. If you try and get a waterproof that goes over the down you'll compress the down anyway, making it less warm, no matter what size you go for. You could go for a monster size to try and avoid this, but it will be mahoosive and unwearable under any other circumstances.
If you are worried about down in the wet, consider investing in some primaloft too, as it works better in marginal conditions and still lofts when damp. I'm assuming that you are wearing the down on belays etc? If you are wearing it when walking in then you probably need to take a look at your layering system!