MrZeroPing, when layering you do need to think about sizes. If you layer over down you need to make sure you do not compress the down or it will lose insulative properties, to avoid this you might need larger outer garmet if it will be worn over down. Same if wearing down jacket in sleeping bag, you need to make sure bag doesn't compress jacket.

In cold windy weather I love my Montaine 3 oz windshirt over a 100 or 200 weight fleece jacket. I did not size up on windshirt when I bought it but if I had to do again I would get 1 size bigger. Montaine's windshirt runs tight and when I pull over my head it is very tight fit getting arms through.
The windshirt can be sized up because it only protects wind and not add real warmth. This will allow it to be worn over bulky insulative layers.

Chuck