You're a bit stuck with Double Breasted, they should ALWAYS be worn buttoned up, there is simply too much material to justify wearing them undone, taking a jacket in won't make any difference to the way it sits. DB is also supposed to be a little bit more boxy, the shape is more classic so it will never have exactly the same lines as a single breasted jacket, that said, there is no reason for it not to be taken in to create a slimmer line, I'm going to be pinning my DB jacket at work this weekend.
I would follow the classic rules of tailoring:
- you should have around 3/4 to an inch of space in the front of the jacket.
- the hem should sit no higher than the bottom of your zip or your actual bottom.
- the shoulders should have about half an inch of space on either side (preferably less, but if you were to lift them up from the edge, it should crease slightly).
- Your sleeves shouldn't go any further than the base of your thumb although you can lift them higher depending on how much shirt cuff you want to show.
A jacket that is too tight is almost as bad as a jacket that is too big. have things taken in but don't go too mad, just remove a bit of the excess material to get the lines slightly closer to your body. This also leaves room for an extra layer or any surreptitious weight gain. It is of course up to you how you want your jacket to look but it is always worth keeping in mind some general rules.
not all jackets are the same either, they might have more canvas, more padding, be cut differently or be made of different material. This all affects the way they will fit.