i wondered what had happened to you ... love the pic of Luna and Zeus.
i would get a size S in ruff ruff couture. my Minnie is around 5 pounds or sometimes 5 1/4 pounds and she is a small. so size S would give Luna some room cause she will probably grow a tiny bit more.
she may fit in an XS but she would grow out of it if she even gained an ounce.
heres a thread of ruff ruff girls XS but i have some of these in size S
ruff ruff couture size XS girls things
the first pic... i have that starlet dres in the middle with the little red hearts
the 3rd pic i have the fairytail dress with the butterflys
4th pic i have Chantal and Darla dress ... they are the one on left and middle
then i also have Ellie dress ( has cute little elephant on it)
an i have lollie love dress ( has lollipop on it )
afternoon tea dress ( has big pretty roses on it )
and trixie dress ( pink velour with skull on it )