Mathematically, the finished block with seam allowances is 14 1/2". It's made from jelly roll strips of 2 1/2" so, 7 finished strips is 14" plus the two 1/4" outside seam allowances.
I am not a perfect sewer and my blocks were around 14 1/4" and I squared them to 14" even. It's my first time using jelly rolls and I don't think I was always consistent on sewing with the pinked edges of the strips. I tried to keep the seams consistent on my squaring up like lining up the seams for the strips at 12" with the lines on my ruler.