I also recommend Celltei. I have the Pet On The Go model and my 5 lb Pixie and my 7 lb Tuffy can both fit in the small size. They are a little cramped in there so I'd go up a size if you plan on them being in there a long time. The Celltei website has guidelines to weight limits and their customer service is great if you have any questions. The Pet on The Go is also super easy to carry since it's hands free and all the bags from Celltei are very well made.
Celltei.com