Maybe a visual would help. She is trying to make the center of the Corn & Beans block. The drawing below is from EQ7. The reason there's an issue with the size is that it should be made from quarter square triangles instead of half square triangles.
The original question was what the correct size of center should be, and that is just a smidge under 6 1/2". The "add 7/8 to the finished size" doesn't apply here because she is not making half square triangles. Hope this helps.