Katie Scarlett is 5.1 pounds and a XS is a teeny big on her, but XXS is too small. I make my own dog clothes and size her to fit just right. Many of the vendors here on YT can do custom orders for you to fit your little one just pefect. Hope that helps some!
Or...you could always pick up the hobby of sewing with us?!
It's so fun! And there are some great patterns out there that you can follow exactly or change to fit your design tastes.