According to Nike Size Guide, Size Small fits Waist Size 29 - 32 inches given that it has adjustable drawstrings.

Channeling the signature details of Nike's Tech Fleece, these Sportswear Tech Fleece Winterized Joggers are made with plush, fluffy fleece for comfort that combats cold conditions. Articulated paneling follows the shape of your body for a less restrictive feel while you move. These pants feature a slightly tapered design funneled down to ribbed ankle cuffs for a streamlined fit that lets you show off your favorite kicks.

Fabric: 73% Polyester | 27% Cotton
Smooth Tech Fleece
Articulated paneling
Elastic waistband with drawstring
Ribbed cuffs
Vertical zip hip pocket
Hand pockets