I didn't realize that this harness was available at a few different places...I've only seen it at Yorkie Couture...is this where you got yours from?...It only comes in 1 size there, X-small, its says it starts at 5 inches and can go to 7.5.....I'm thinking this might work..Mackenzie is only 1lb 5 oz and 12 weeks....so I would think x-small is what she needs?...let me know what you think, being that you already have the harness you can be a better judge...thank you!