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.
Boys 3D Graphic Hawaiian Aloha Short Sleeve Dress Shirt Tops with Button Down - Sizes 2-8 Years by Uideazone
47 Review
Star Wars, Marvel, and Disney Short-Sleeve T-Shirts for Boys from Amazon Essentials
50 Review
π Hanes Boys' Short Sleeve T-shirt Value Pack (3-pack) - Essential Wardrobe Basics
44 Review
Van Heusen Sleeve Dress Poplin
42 Review
π Bold Pack Sleeve Active Hoodie Sweatshirt Set for Boys' Clothing - Stylish and Comfortable!
47 Review
Funny Guy Mugs Tearaway Sweatpants: Playful Boys' Clothing for Fun and Active Days
49 Review
Waterproof Cozy-Dry Rain/Snow Pants for π§ Kids by JAN & JUL, with Fleece Lining
64 Review
π Fruit of the Loom 6 Pair Cushion Ankle Boys' Clothing: Comfy and Stylish Selection for Active Boys
57 Review