But, there are ways for you to find a short list of bags that ought to fit. Bag makers almost always list a "girth" measurement for a bag, which measures its circumference at the widest pointusually (but not always) the shoulders. L.L. Bean's very fine Summit 775 20 ($235), for instance, measures 56 inches around the shoulders. That's pretty trim, but a trim bag saves weight and makes more efficient use of the insulation, so there's logic behind that.
By comparison, Marmot's Wasatch ($195) measures relatively generous 63 inches at the same point. Wider still is Western Mountaineering's Badger, a bag designed for wide people that has a whopping 66-inch girth. Alas, the Badger is rather a pricey bag at $375, though worth the price as Western's bags are incredibly good. Feathered Friends' Penguin is another good-sized bag (64 inches girth) and also very high quality. It's $300.
The alternative is a semi-rectangular bag, which is much less tapered than a mummy but not as boxy as a fully rectangular. Sierra Designs makes a pretty good bag of this type called the Napsack, rated to 24 degrees, down fill, 69 inches at the shoulder. And weight is not too badtwo pounds, 11 ounces. For all-around camping, a good choice.