How do I make a hexagon in a large size(14")?

Draw it out on (freezer) paper and use that as a template? If you have a quilting ruler with 60 angle lines on it, or a protractor, you should be able to draft the hexagon easily enough.

Cut six 7.5" (close enough) 60 degree equilateral triangles. Sew 2 together, add a third, do that again, then sew the 2 halves together. Done. ;)

Or did you want the hexagon to be one piece of fabric, not pieced?

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Check out the Internet, they have free downloads of any shape you want, from circles to squares onwards. Then download what you want, and most modern printers will enlarge or shrink them. Or cut in half, enlarge that and add seams to it.

Riversong has a tute on how to make one, I am sure you could adjust the size. I am sure you can find it doing a search