I have a very small 5.5# cat and while she does fit in the small carrier, she is more comfortable in the medium size (which I bought for my 12# cat).  The small carrier fits easily under a standard airline seat.  For the medium, I had to remove the wheel base (easy-peasy) as it was about an inch too high (Alaska Airlines Coach seat, which are relatively generous).  Love it and would most definitely recommend it!