Okay, so I currently have 11 pattern sections of the shawl done, that is 11 of the 10 row repeat. Having measured its current size, and looking at the pictures of Kate's shawl, I am thinking that a total of 15 10-row repeats will give me about the right size, based on my size. I am 5'6" tall (not nearly as tall as the lovely Catherine), so I think mine will end up being a bit smaller than hers.
I am estimating, without the ruffle, that this will take just over 2 skeins of Rowan 4-ply soft. I think the ruffle is going to eat up quite a bit of yarn, but I haven't done any samples yet.
Oh, and with my current gauge, the 15 repeat size of the shawl will be about 64" tip to tip and 26" deep without the ruffle. I think I will cut out a fabric shape of that size to make sure it is what I want.
And here is a funny for you. Tonight, while at a birthday party for my nephew, I was asked what I was making. I told them a shawl. They replied that they like the stitch pattern, etc. I said thank you, I had seen a pic of Kate Middleton wearing a shawl and decided I wanted to make something similar. At this point, my SIL's brother said. "Is that how it works? You just look at it with your eyes and it comes out of your hands?" I laughed and said yes, something like that and his mom said, "Yes, Jess, it's MAGIC!"