For Olivia's quilt I made the blocks 4" finished, and below is a diagram of the sizes of the pieces:
To get a finished block of 4" the easiest thing to do is cut rectangles of 4.5" x 5.25" and then cut the diagonal line along the middle. For this example I cut my diagonals at 1.5" in from the two opposite ends of the long edge - you can do this by either marking the edge at 1.5" in, or what I tend to do when I'm cutting lots or something I put a couple of little marks on my ruler using a sharpie to guide where to place the ruler to do the cut with the rotary cutter.
If you wanted to make a bigger or smaller block, but with an angle about the same as this one then you just need to cut a rectangle with one side 0.75" longer than the shorter side. If I'm doing something and am not sure about the size, I often draw it out on paper and measure it to be sure, so if you want to test it, that's a good way to go about it, much better than wasting fabric by cutting it wrong!
Alternatively, the way I did the wall hanging is to cut a square on an angle, then trim down the blocks once I'd sewn the diagonal seam together. I did this because I had some squares left over that I'd pre-cut so I used them. This would probably also be a good way to go about it if you were using charm squares. Make sure to cut the two sides equally so that the whirly-gigs don't end up uneven! (Unless that's the look you're going for of course)
A very brief tutorial of this today, but hopefully enough to encourage you to give this design a go! As always, please let me know if you have any questions, want any clarification or assistance with working out sizes for these please get in touch. And of course, if you make one of these please share a photo with me!