Certainly armspan is a wonderful place to start.
To me, it depends on how you anticipate fastening it around yourself. Do you want to tie the ends into a knot? Do you want to fasten it with a broach [sp?]? Will it rest in the crook of your elbows?
And do you want the shawl long enough to sit on the center tip? To go down to your knees?
This is all a matter of personal taste. Some like long graceful shawls, others are happy with just-below-shoulder-blade length.