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A mummy sleeping bag has a tapered shape and usually a shaped hood. Choose it because that fit works for your sleeping position and trip, rather than assuming every mummy bag is warmer than every rectangular bag.
Try the shoulder and foot space
Check the length, shoulder width and foot area together. Bend a knee and turn onto your side if that is how you sleep. A bag that looks roomy when laid flat can feel restrictive once zipped. If you are near the upper end of a size range, try both sizes where possible.
REI describes mummy and other sleeping-bag shapes and notes that fit differs between models. A semi-rectangular shape may be worth comparing when you want more movement space without moving to a full rectangular design.
Hood and collar adjustment
Lie inside and try closing the hood. You should be able to find and operate the controls without creating uncomfortable pressure. Check whether the selected model has a separate draft collar and how it works with the hood opening. These are individual construction details, not guaranteed features of anything sold as mummy-shaped.
Zipper and ventilation
Operate the zipper from inside. Look at snag protection and any draft tube behind it. A longer opening may make entry and temperature adjustment easier; a shorter zip can change those functions. If warmer-weather use is part of your plan, check how much of the bag can be opened without fighting the tapered shape.
Temperature and packing
Read the rating independently of the outline. Match the needed warmth and sleeping-pad insulation before comparing weight. Then check the compressed volume against your backpack. A close-fitting cut can be attractive for carrying, but weight savings should not come from an unusable size. Follow the selected bag's cleaning instructions and avoid leaving it tightly compressed between trips.
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