Hi Jean!
For Darn Tough Jr (meaning children's) socks, an 11 would be a small, and a 13 would be a Medium.
If you are looking for adult sizing, the men's 11 would be a large, and 13 would be an XL. You can also find our size chart to the right of the price on the product's page. It will be small blue letters that say "Size Chart" that you should be able to click on to bring it up!
If you have any additional questions, feel free to either send us an email at hello@gobros.com, or give us a call at 866-446-2767, option 1. Our office hours are Monday-Friday, 9:00 am - 5:00 pm, CST.
-GoBros.com Customer Service