Any fiber needs about 1" in an overlap at break like the end of the roll of cloth.
It is a serious waste of money to do carbon fiber if you don't at least vacumn bag.
I don't work on Dreamliner, but on other aircraft there are no scarfs on original manufacture of a carbon fiber panel. The only time you will see a scarf is in a repair and that would be avoided if the panel can be removed and replaced.

You have to realize that there might be 10 to 30 plys on a skin. If you had to stop a ply because the panel is too long, you would never put an overlap splice at the same place in the rest of the plys. So thru the thickness you would never have more than 1 splice thru the 30 ply thickness at any place.