How do you size your rain shell?

My Large size H2NO Patagonia jacket has proven simply bombproof above the tree line in a storm.  It fits over my UL MontBell Down Jacket and a base layer.   If you are likely to encounter survival conditions in Winter I think you should be able to fit all your insulation under your hardshell jacket.  

Jim, What’s your actual size? Looks like your Montbell jacket is on the thin and light side.

I typically wear a size L jacket and I’m 6 ft tall 170 lbs 41 in. chest. A size large will give me space for a baselayer, midlayer and a light puffy. My shoulder season and winter down jacket is a Patagonia Fitz Roy Parka 8 oz down version – if I squeeze that under a size L jacket it will not be able to loft to its full size. An XL jacket will leave me a lot more room but a lot of extra space when only wearing a base layer or base + midlayer, and the sleeves will be too long.

[In winter we also carry a 2P bothy/emergency bag – a light waterproof bag big enough to pull over two, with an inner edge to sit on, a small clear window and air tubes for ventilation. Fantastic for quickly getting out of the wind and rain. It’s much faster to deploy and put away than a typical shelter, so handy for breaks or an emergency].