I figured since it was going to be big and loose that measurements weren't so important and I could probably wing it. At one point when it was too wide for me and the sleeves were still full-length and loose, I was quite discouraged with how much it looked like a judge's robe! Not sexy. But then I made a few adjustments and the shape came out much better; I cut the sleeves shorter, put in cuffs, added three long pleats in the back to bring the shoulders closer together, and rounded off the bottom edge.
I also put in binding along the entire opening edge to help it hang nicely in the front. Finally I added some detailing with ribbon, just like with my cushions and Boxing Day shirt. I was ambitious enough to use the zigzagger to sew in the floral ribbon pattern in the back of the shrug. That was pretty tedious and not necessarily easier than hand-stitching but it was good practice!