It would help to know what pattern you are using. Try
http://www.quilterscache.com You may find your pattern in 12" there. I am a visual person, so I the draw the block in the original size, figure out the size of the finished (pieces no seam allowance). Then I figure the size needed to fill in the spaces for the larger block and add the 1/2" seam allowance. If that doesn't work for you, PM me a photo of the block and I will give it a try.