What size is this T-shirt?

It is labeled small. The seller lists a 20-inch armpit-to-armpit width and a 27-inch collar-to-hem length, so compare those to a tee you already own.

It is used and described as in good condition. Expect some signs of wear, and check the measurements rather than assuming a new fit.

The material is not stated. New Balance tees of this style are usually cotton or a cotton blend, but confirm with the seller before buying.

How does the fit compare with a new New Balance tee?

The listed measurements are the reliable guide. A 20-inch chest on a small is fairly standard, but used shirts can shrink, so measure a tee that fits you well.

Can I wear it for workouts?

It is a casual cotton-style tee, not a moisture-wicking performance shirt. It works for light training and everyday wear but will hold sweat more than technical fabrics.