Unless you know that your local seamstress is able to do work like this, I would not recommend letting her cut up your sleeping bag
My experiences with this type of service are, that misunderstandings and errors happen very easily. I'd rather have a fully functional, slightly oversized sleeping bag than a mess that I cannot use anymore. I also suspect that it's not just a matter of cutting excess material and sewing the bag back together. In my case, the zipper would need to be exchanged as well (I think it's not possible to simply cut zippers).
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