There are a lot of great websites out there to help us quilters with some calculations that are needed in everyday quilting. Some of these include how big of a square to cut when you need to make half or quarter square triangles. One of the sites that I tend to visit is that of Karen Combs.
One of the basic calculations that I often refer to is on Half- square triangles. Here is how Karen describes it:
Half-square Triangle: Add 7/8” to the height and length of the finished size of the square made from sewing two half-square triangles together. Cut square in half, diagonally to create two half-square triangles.
So if I need a 5 1/2'' finished square, I should add 7/8'' which totals 6 3/8''. I draw the diagonal line and stitch a scant 1/4'' for a resulting 6'' unfinished square.
Here is my starting square:
And here is my resulting half-square triangle after pressing:
Slightly larger that 6'' so I can square it up.
Another great post to read is this one by Generations Quilt Patterns. Here they give you a couple of options for making the HST units, one similar to the above method and the other starting with oversize squares. They also give you some suggestions if your HST units just aren't coming out the correct size.