UKH

Looking to buy an insulated jacket as a throw on when stopping for lunch or belaying. Are they sized to go over layers and shell or do you have to go a size larger?

I've found they expand enough to go over fairly thick layers. I'm a medium and my medium Mountain Equipment Fitzroy goes over everything just fine. I've got a lighter Montane Prism that is the same.

I've bought jackets way oversized, they turned out much better investments as they go over everything and fit all your mates. Smaller ones just get compressed so are never as warm. If in doubt go bigger 

It's case by case I think. If it's marketed as a Belay Jacket I'd expect it to be roomier than if, say, it's marketed just as insulation. It's worth asking if you can't try on.

My experience finding a belay jacket was that the armhole size was trickier to get right than chest, waist or length. The shoulder cut of any shell you might wear underneath will have an impact.