Why don't don't you go ahead trim it down to 12.5" ?
For others wondering about resizing Anita's Arrowhead, please check p6 of Instructions and Patterns, after clicking on the Materials tab for the PDF downloads
It is 12 1/2" except the triangles on the sides are shorter :-(. I thought I might trim it to 12" then put 1/2 border around it. I just love this pattern!