I bought size intermediate for my slim 6 year old. This top has 2" (!!!) smaller elastic band than her other brand's size 4-5 we have.
From the sizing info: Intermediate size (6-8), bust 24-25". However the (very firm) elastic has circumference 17.5" (!!!!!!) at rest, 18.5" comfortably stretched. My 6 year old's measurement around ribcage is 21.5" and chest 22.75". The elastic band is extremely tight for her, making it impossible to wear.
If not the sizing, this would be fantastic quality product.
I really like the adjustable straps and the fabric of the top. My daughter is only 6 so I ordered a small. This small is really small. I know that a medium would be to big for her. If I order more, I would wait until my daughter is older.