I'm really far from Pasadena! hehehe....I bought it at a place called
"The Red Hound" on College Ave in Oakland. Here's the Celltei official website. I think they may have all the answers to your questions:
http://www.celltei.com
This is a more direct way of getting to the site:
http://www.celltei.com/petonthego.html
The one that I have is called "Pet-on-the-Go."
If your original bag is pretty spacious, and your dog is used to more room, I would probably get the medium.
On the other hand, if your dog is used to being in a tighter bag, he might not mind the small.