I believe it’s a large the size has worn off but it fits like a medium.

This stylish blue racerback tank T-shirt by Adidas is perfect for any sports enthusiast. Made from Jersey fabric, it is breathable and machine washable. With a solid pattern, crew neckline, and pullover closure, this sleeveless T-shirt is perfect for summer. It is a size M, with an athletic fit for both men and women. The T-shirt has a logo accent but no graphic print. It was manufactured between 2010 and 2019, but it is not vintage. The T-shirt has a theme of sports and features Climalite technology to wick away sweat and moisture, making it ideal for workouts. It is a regular size and is not personalized.