Sometimes I only want half a sheet of frozen pastry, bending and snapping does not always have a great outcome and does not always leave you with usable pieces.
The only way I’ve found that works for me is to get a long bread knife (you need the serrated edge) and very carefully saw along where you want to snap. Only go about half way through, top to bottom, then carefully snap.
Of course this only works for straight lines, this will not work for curves.
Pop the unwanted half of your still frozen cut pastry sheet back in the freezer straight away to use another time.