Hey @cdrmb - Thanks for bringing this question to the community! It'll be helpful to know if you plan to use the sleeping bag for car camping and/or backpacking, as the weight/bulk of a car camping bag can be difficult to accommodate for backpacking.
For shoulder girth, a ‘typical’ mummy bag measures between 58” - 64”; though there are some companies, such as Big Agnes, that tend to offer a bit more space.
If you're not planning to backpack, you might consider the REI Groundbreaker 30. It’s a rectangular bag with a shoulder girth of 66”, which should be a comfortable width for most people looking for a bit more room.
If you're planning to backpack, and weight/bulk matter, we'd recommend the Big Agnes Torchlight 30. It has expandable panel down each side which allows you to configure the shoulder girth from 60” - 70”. With the ability to open/close the zippers in various positions, you can really dial the shoulder/hip spacing that works best for you.
We hope this helps get you started; let us know if you'd like additional suggestions.
@cdrmb thanks for the response via email! Since it sounds like you will be backpacking with the sleeping bag, and thus weight and bulk matter, here are a few options to consider in addition to the Big Agnes Torchlight:
Let us know if you have additional questions!
I had a similar problem (for years). Not only are my shoulders wide, but I tend to move around lots while I sleep, and I never liked being sealed in a bag. I finally realized that the problem was not in my gear but in my head: I switched from a mummy bag to a quilt, and bang!, problem solved! Before I went full-on quilt, I made use of my 20-degree bag in a "quilt-mode" by unzipping it all the way to the foot box and using it as a quilt. What, you might ask, did I sleep on with my mummy bag-turned-quilt? I slept right on a short closed-foam pad that I place on top of my air mattress. Since I always wear a light shirt (or even light thermal) when I sleep, I am never really sleeping directly on my shortened Therm-a-Rest pad.
I've had similar problems. The usual way to go is to get a large rectangular bag. You can always unzip a bag (that can unzip flat, or almost flat) and go "quilt style". If you want something that's more mummy style, then you're going to have a challenge finding enough width. I couldn't ever find one that would fit my shoulders for backpacking. Then I found Enlightened Equipment! They offer multiple widths, and I got one of their "Convert" models in an extra-wide width. Problem solved!