You mentioned "pieces", which makes me think that you might be making an envelope back -- you need to add 4" to the measurement of the uncut front (13.5+4 = 17.5) and then cut that in half (17.5 / 2 =
8.75) so both of your pieces for the back would be cut at 8.75". Your finished pillow would end up at 12.5" because you take a 1/2" seam.
Because 12.5" is going to be nearly impossible to find, I would take a 5/8" seam & when you're finished, top stitch 1/8" inside of that around the outside when finished. That would get you down to 12".
Alternatively, you could just cut your pieced square down to 13", cut your two back pieces at 8.5" and use a standard 1/2" seam throughout.
http://www.marthastewart.com/270503/...a-pillow-cover
This one has diagrams (note: it includes the fabric for the front, which you already have):
https://thedecorguru.com/2012/12/11/...-and-tutorial/
Also, even though I am generally a fiber purist, this is one time where poly thread really comes in handy. The extra strength will ensure your pillow lives a reasonably long life. I also would shorten the stitch length for the same reason to 1.4-1.6mm.