I've made lots of those, in red white and green for Christmas gifts. I have done the 3 needle bindoff, where you hook through one of the stitches on the original cast-on edge, as you bind off your final edge. It makes a ridge, but I liked it better than having to stitch it up. Just look carefully at what matches with what, so you get the points on your pinwheel right. I can't advise on making them bigger, because I don't know how the points would turn out, and if it would lie flat etc.