The web site states up to 6 lbs but realistically I think 5 lbs would fit better - the pup would have a bit more wiggle room.
It's a nice, sturdy bag, good looking with a removeable, padded bottom inside, pockets to keep things in, a leash clip, zip/roll down half windows, a place for a water bottle and another spot where you can put a small cooling unit for warmer weather.
One very nice feature is the car seat belt can slip through special openings on the back so it can double as a safe car carrier.
Thanks for asking, if you have anymore questions, let me know!!