I hope this will help: I find the size small fits most bags. I have a few very small bags that even the size small would be too large for and I have one very large bag (Anna Corinna drawstring bag) that the small kind of tips over in. I solved that problem by also ordering the large sturdy Chameleon, leaving it in the bag, and when I use the bag I just place the small Chameleon inside the large sturdy one. In addition, I use a huge Vera Bradley when I travel and the small Chameleon goes right inside and I put other traveling things (like book, magazine, sweater, etc.) around the Chameleon. I purchased a Chameleon for my mother for mother's day, and I chose the size small for her too. Good Luck, I think you will love it.