What is the folded size of a sectional map?
One seller's link on Amazon claimed 10" x 5", but when I measured an old one (only one I have handy), it was between 10.25" and 10.5" long by 5" wide. So I'd recommend using the latter value, knowing that the chart will probably be a hair under 10.5" long and so you have room for error (or a badly refolded chart).