First, let's make sure we are talking about the same block, since I have met about 6 or7 different "Ohio Stars". I am attaching what I have most often seen called an Ohio Star. If you look at it, there are different sub-blocks. For a 12" (finished) block, each of these is made at 3.5". (3" finished size). The math rule is
1. subtract the seam allowance.
2. multiply each unit to the finished size you want, in this case, you want it half again as large, or 4.5" finished.
3. add the seam allowances back in, so each sub-unit will be 5".
If you are making the center sub-unit as one larger square, instead of 4 smaller one, as I have shown for convenience, the same rules apply for it. Original cut size is 6.5" Change to 6". Increase x 1.5-> 9", add allowance and cut at 9.5".
Other people have other methods for making HSTs. I add 1" to the finished size I want, mark the diagonal, sew 1/4" to each side and cut on the marked line. I then press and trim to the exact size I want. I use my ruler's 45* line to make sure I get it exactly right.