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My expirience so far is that a 3/4 underquilt can be narrower because it will follow your body better than a full length. I just made a 40" that seems to be working great, and think a 35" wouldn't be unreasonable. The smaller the width the more the suspension may be in your face, though.
My width measurment is insulated width, and doesnt include suspension channels.
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I'm also 5'3" and I have the short incubator. Honestly I'm sure I would have been fine with an even shorter UQ (maybe even 3/4) as I sleep fetal a lot of the time. But I'm grateful for the entire width. If you move around much in the night, it's good that the new spot is insulated. You'll sleep better of you don't have to move the insulation or try to stay right on top of it.