I am 24 years old. 165 pounds. 5'9. I usually go for a large boy size but after reading the reviews and seeing some people saying they were too big I decided to size down to a medium. 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.
π Hanes Boys' Short Sleeve T-shirt Value Pack (3-pack) - Essential Wardrobe Basics
44 Review
Van Heusen Sleeve Dress Poplin
42 Review
Kid Nation Kids' Tagless Long Sleeve V π Neck T-Shirts 4-12 Years - 2 Packs and 3 Packs
45 Review
Star Wars, Marvel, and Disney Short-Sleeve T-Shirts for Boys from Amazon Essentials
50 Review
Funny Guy Mugs Tearaway Sweatpants: Playful Boys' Clothing for Fun and Active Days
49 Review
π₯ Stay Warm and Active with Rocky Fleece Thermal Underwear Bottoms for Boys
44 Review
Arctix Youth Pants Reinforced: Large Boys' Clothing for Active Adventures
46 Review
Boys' Adidas Active Sports Tricot Athletic Clothing
45 Review