Sully is 13.5" from neck to tail. He is also 9.1 pounds, fully grown (8 mo. old).
But when I bought him a SMALL hawaiian shirt this week, on eBay, it was way too big for him. Still wearable....but I had to use safety pins...oh well, live and learn.
Good luck. I would say definitely buy from a place that would custom fit your dog's size. And remember that your baby will grow, and fast, so if it's a tad too big, it's ok, for now.
www.barkindogclothes.com has some really cute stuff, I've really been admiring from aimee's posts.