Ah, LeMoyne Star. That makes sense.
Here's a picture of a diamond, wrong side.
The size of the square should be the length of the side of the diamond. This diamond shape is a rhombus (equilateral parallelogram), so all the sides are equal.
First mark a quarter inch line from the edge, and then measure between the 2 intersections (I've made tick marks). Add 1/2" for a seam allowance.
If you measure tip-tip, you'll get a slightly longer length because of the point. And then you'll be easing the excess in when you sew the block.
And that's all I've got. Good luck. ;-)