This is a great little jacket. Both of my boys love them and they appear to be made with quality product and sewn together well. Let alone the boys think they look really cool. I struggled with the size info, so hopefully this might help someone else. I ordered an Extra Small, Small, and Medium. I am keeping them all because the boys will grow into the Medium.
The Extra Small fits my 5 1/2 year old perfect. He is about 45 lbs and average size for a 5 year old. He has been wearing size Small typically.
The Small fits my 7 1/2 year old and is a tiny bit big. He is 55 lbs and tall and thin for his age. He has been wearing size Medium Slim typically.
I am a tall slim size 4 woman and the size Medium fits me perfect except for the arm length, but I have long extremities. I would wear this jacket if the arm length fit.
To summarize (at least for my family) we went a size down for everyone from what they typically wear.