I am 24 years old and weigh 165 pounds. 5'9. I usually go for the large boy size, but after reading the reviews and some people said it was too big, I decided to go down a size. The seat was tight but comfortable. I needed it to be tight because I was wearing it with a jumpsuit. So everything turned out perfect. Well if I made this shirt with jeans I would make a big one. Here's a little detailed review to help someone contemplating sizing as some of the reviews I've read have been helpful and some have not. Hope that helps.
Star Wars, Marvel, and Disney Short-Sleeve T-Shirts for Boys from Amazon Essentials
50 Review
Soft Brushed Fleece Crewneck Sweatshirt for π Boys and Girls 4-12 Years - Kid Nation Kids
42 Review
The Good Day Lab Clean Shield: Ultimate Comfort and Breathability in Tagless 4-Way Stretch Polo
43 Review
Boys 3D Graphic Hawaiian Aloha Short Sleeve Dress Shirt Tops with Button Down - Sizes 2-8 Years by Uideazone
47 Review
π₯ Stay Warm and Active with Rocky Fleece Thermal Underwear Bottoms for Boys
44 Review
Waterproof Cozy-Dry Rain/Snow Pants for π§ Kids by JAN & JUL, with Fleece Lining
64 Review
Arctix Youth Pants Reinforced: Large Boys' Clothing for Active Adventures
46 Review
π Fruit of the Loom 6 Pair Cushion Ankle Boys' Clothing: Comfy and Stylish Selection for Active Boys
57 Review