in addition to asking what block are you going to make, my suggestion is that you clarify whether you really need to change the block size by only a half inch. It almost sounds like the block you need is to finish at 12 inches, which means it would be a total of 12 1/2 inches unfinished...or before you stitch it to surrounding blocks.
I've never seen a requirement for a block finished to 12 1/2 inches...however, 12 inches finished is a common size. Does your pattern clarify finished size vs. unfinished size?