Here is a little bit of information that may help. The link that katier825 shared tells you the size square to start with depending on the finished size of the block. The first time I made a quilt that was set on point I had a wonderful teacher that explained why the setting triangles need to be cut differently. If "the why" is explained so that I can understand the reason, I am able to figure out the size.
The thing to remember when setting any quilt, whether you are working on a horizontal quilt or a quilt on point, is you want the "straight of grain" to be along the outer edges of the quilt. You do not want a bias cut along an outer edge. Bias cuts are more apt to stretch than "straight of grain" cuts. To achieve "straight of grain" along the outer edge, you will need to make two differently cut triangles.
To make the side triangles, you need to use a square cut diagonally twice using an X from corner to corner. That puts the hypotenuse of the right triangle (straight of grain) on the outside edges. Since you will get 4 triangles from each square, count how many triangles you need and divide by 4. Round up if needed. For your block that you say is 5 3/4", I would start with a square that is 9 3/4" or even 10". Cut diagonally corner to corner both directions (cut like an X) making 4 triangles from each square.
To make the corner triangles, you need to again have the "straight of grain" on the outer edges. To do that for the corners, you need to cut your square the size you need and cut once from one corner to the opposite corner on the diagonal. Since you will get 2 triangles from each square, you will need to start with 2 of these squares for one quilt top. You will now have "straight of grain" on all the triangles around the outer edge of your quilt. For your 6" block, I would start with 2 squares that are 5 1/4" or 5 1/2" each. Cut diagonally one time from upper right corner to lower left corner. After you have all your triangles sewed in place, you will need to trim remembering to leave the 1/4" seam allowance at the points.