I would replace only the wet drywall and wet insulation, cutting back the drywall to a joist for attachment. I would certainly go bigger (for ease of attachment) than what fell but I also wouldn't go to more that 4'x8' unless it was wet beyond that.
Oh, and fix the roof first.
Also - personally, I wouldn't get the insurance co involved for such a small project. No need to make it into a big project IMO.
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