It's been my experience that puppies grow a lot in how high and long they are the first 6 months and then they start to put on more muscle and fat later on. Your dog has still got some growing to go. Make sure you buy a bag with a firm bottom on it. Some of them sag from the weight of the dog if they don't have the right bottom.
Gracie prefers to have room to turn around in her bag and does not like to have to squish down to have the top zipped around her. The Zack and Zoey that appears above is nice because it is some what taller than many bags. I know some people prefer the Quigy Bo.
Walmart.com: Backbone Pet Jane Pet Carrier: Dogs
This is the latest of my collection and is one of my favorites. It has plenty of space and headroom with storage places and room for her favorite blanket. I have seen it for less on other sites so if anyone likes it shop around.