I think I'd measure from edge to edge around the curve of the poofy part of the pillow to see if 12 1/2 will be enough. Make sure you make an allowance for the seams, too, though.
"Trust the pattern!"
Time flies like an arrow, but fruit flies like a banana.
Hop aboard while we follow the cloud-covered van of knitting harmony, man!! Woooo!! --Hedgehog
IngridKH on Ravelry