I have the medium size one (the one that is 15 inches long), and Luna is 4 pounds. I tried first the small size and it was too cramped (sp?), so I decided to get the bigger one... I have the bowler style, and I love it. It is very comfy for her, sometimes a little bulky for me, but it is worth it, as she is so happy inside it. If your pup is still growing, I would for sure get the bigger one, unless you are planning to change it soon!!!
By the way, today we sneaked into HEB, it was too hot outside to leave her in the car, and I really needed to get some stuff, and it worked perfect! It is very good to enclose them totally, but not so good to have them sitting... anyways, I love it, the finishes are the best, and is SOOOO cute!!
Forgot to add, I got mine at Neiman Marcus... didn't want to take the chance to go thru EBay... I got a little one, even when I asked for the big one, and ended up paying more money that what was supposed... I learned, so I went to the big store... and if you are lucky you could get it on sale!!