Ah! So in this case you want to attach the corner triangles cut at right angles (no bias) so you would start with two squares that you slash along the diagonal. For me the simplest and easiest way is to cut large and trim down to desired size. With that in mind, I'd use the same size squares as what I am framing, that is, with 8" frame, I'd slice an 8" square in half and have quite a lot to trim down (about an inch or more).
Otherwise, there is math involved and there are charts
Here's one site:
https://www.generations-quilt-patter...triangles.html
My web quilter has a lot of help, here's the portion for exactly what you want:
https://mywebquilter.com/CalculateSquareinSquare.aspx