If the finished size of your block is 12", multiply 12 x 1.414. It's the square root of 2 and a geometry thing. That equals 16.968. Round up. I like to go up and make them oversized and trim later, so I would cut squares about 18". Obviously, you will get 4 triangles per square, so if you need 30 triangle for your quilt, you would cut 8 - 18" squares
Cut each square in quarters diagonally. That gives you a straight-of-the-grain hypotenuse for the edge of your quilt - VERY important for a non-wavy border.
For the corners, I usually just cut two squares in half diagonally - for this quilt, a 10" square would work fine.
After you sew your quilt together (you will do it in diagonal rows), you will use a long ruler and rotary cutter to trim off the excess triangle fabric. I actually like to leave a little more of the triangle fabric so I don't have to make my border meet the points perfectly, so I might trim at 1/2" or more instead of measuring and cutting 1/4" from the block points. That causes the blocks to "float" in the body of the quilt and prevents their points being cut off.
Hope that helps! :mrgreen: