I got my baby a 'nervous' harness, she's about 6kg and we got a size small - could have gone an XS maybe as the small is on the tightest it can go and a little loose, but still wearable! It's great quality and really helps people know to not pat her in public so she stays a lot more calm.
My dog Ruby has the nervous lead, harness and collar. I’m so glad I finally bought it as it tells people not to come next to her and not to bring their dogs near her too. This allows Ruby to enjoy her walks peacefully!