Once I started sewing it became clear that the blue fleece should be the lining to provide comfort and warmth -- and the plaid for the outside. The blue fleece was better for the contrasting scarf. Here's the finished coat.
Maybe I don't know how to measure dogs, but I thought his length is 14" and his girth is 16". I had measured around his belly and from the pattern envelope it would seem that the Large is the correct size. It seems too big around, but we accommodated that by trying it on him and then deciding where to attach the velcro.