I think I might have to do what you did with your black BB... Khloe is 5 lbs, but is long and lean and has a small chest. She is in a two right now, but is on the second to last hole, and the harness is a lot thinner. Couv has a 3, and it fits him great... But K is way too small for it
I'm thinking a 3 with extra holes might be the only solution!