I have a 5 pound Yorkie and bought the medium because I read comments that they run small, I am happy with the size I bought, I did not want her crammed into a small area and this is the perfect size for her, there is lots of netting for air to flow through and for her to see out, we have not used it on a plane yet but that is why we purchased it, I will never leave her home when I travel again! I tried that once and I missed her way too bad!