i would end with a blue outside border of about 2". remember you have a binding also. i would go with whatever there is enough of from those colors. first choice being blue, to keep it invisible. if you don't have enough of anything, strip them together so the colors change.
the middle border depends on what you want as the finished size. i think it could be up to 8" and still be very pretty because it's 'enclosed' in the two narrow borders.