Bit late to the party this time, but Blackadder nailed it. You'd just grab one and call it good. The older types (blue ones with US NAVY across the back) were more personal outfitted, though, as you can see from old pics. Those fit the individual sailors better. Some officers seem to have had their jackets tailored.Forget about what the fit was like in those WWII photos. I have read that they are often left on hangers by the hatch so whoever goes up to the deck can grab it
But to say that these jackets were generally worn loose is not supported by WWII photos I have seen. I think you'd avoid a too roomy fit in any kind of work jacket on deck. It was drafty on a flight deck and you did not want to get snagged with a too bulky jacket. As long as you were able to move freely you'd prefer a close to body fit. The high armpits added a lot to the freedom of arm movement.
After the war these jackets were extremely popular as allround purpose outdoor jacket, worn by many (just like the Vietnam War M65 or the German BW Parka). Pictures showing them being worn by civilians show, that they privately were preferred more roomy fitting. Just my two cents.
I pretty much agree to Bfd70. It is good if the jacket does not only fit over a T-Shirt but allows at least one additional layer like a light sweater.