Depends on the max beam of your boat. What are you building?
You can scrounge some more material with a clever layout, but the safest bet is to measure the gunwale-to-gunwale distance at deck level, then add twice the gunwale to chine distance, then the bottom width, plus one or two, or up to 6 overlaps depending on how many seams you will cut.
It generally averages a 6 feet width or a few inches less for a beginner's kayak.
The more skilled and slim the paddler, the more cloth saved.

Eric

: OK, I know my skin has to be at least a foot longer than the frame, but how
: wide does it have to be? I notice the different materials come in
: different widths. I dont want to buy more than I need, but I sure dont
: want to come up short either.

: Thanks,

: John