I have the Alex, the large one and I will say there was more than plenty of room for Olivia - she was 5# and not tall either. I wanted to have plenty of room if I wanted to put a blanket on the bottom of the carrier. Personally I am glad I have the larger one.

It also works fine for my new maltese girl who is a little over 5# but seems longer than Olivia was.
One thing for sure - you will love the bag, QB's are awesome (IMO).