I have made those blankets, and I have sewn them before washing. Instead of 40" square and 30" square, I made them 43" square and 33" square. I washed them when they were done and they came out fine. You really can't made them any bigger than 43" and 33" so they will come out the same size as washing the fabric first. Maybe bigger.
One hint though -- I found that things sewed better if I kept the fabric oriented the same direction. Mud?? Sew the WOF(stretchy) of the inside square to the WOF(stretchy) of the outside square. And of course the LOF of the inside to the LOF of the outside. I also left the turning opening on the LOF side, so that they didn't get stretchy when trying to sew the opening closed. Everything seemed to go together easier when I made sure of this. Have fun! Ann