My daughters friend got this costume for Halloween and she HAD to have it!  My daughter is 8 yrs old and typically wears a size 8/10 shirt.  She's not overweight AT ALL.  Her friend got the Large and we tried that on and it fit so we got the same size.  When she got her costume it actually felt a little tight in the top.  Hopefully that helps you determine size a bit. Looking at their sizes, I may not have ordered the large.  The costume is VERY cute!  The sunglasses and headband didn't last too long because it got dark and then cold in WI.  She said the headband was a little too tight after a while to wear over the hat but whatever...I didn't really care.
Quality wise, I would probably go 4 stars on this but it's still in good enough shape that she can wear it next year if she wants and if it still fits her!  (Course we'll have to stretch out the elastic at the top!)