Assuming, it's a 60 degree triangle, if you sew together three same-sized triangles together, you get a half-hexagon.
As long as you use the same size throughout the quilt, all the pieces will go together smoothly.
If the whole quilt is made of half-hexagons, just pick a size and go with it.
eta:
Sometimes when a measurement is quoted for a hexagon (or half-hex), it's based on the length of a side, sometimes it's across from flat top to bottom, sometimes it's across from point to point.
Which one is your pattern using?
Last edited by MTS; 11-11-2012 at 02:34 PM.