That is a 5 patch style block and it can be increased to 12" but it gives you some really funky math. Is there any way you can make it at the 10" the pattern calls for and just add a 1" frame all around?
My calculator tells me that you'd have to cut the 2.5" squares as 2.9". You might get it to work by cutting them at 3" and adjusting your seam allowance a little more than (a smidge to the left) 1/4". Cut your half square triangles an inch larger than it calls for, assemble into squares and trim them down to the same size as the squares and then sew them with the same plus size seam allowance. It's going to take some finessing to get the blocks to the perfect 12.5" unfinished size but it can be done.