I have a 32 inch waist size and wear a size medium sweatpants.
Sweatpants come in Medium, Large and XL.
The Medium size sweatpants fit a 32 inch waist and a bit smaller as well.
Also the medium size sweatpants also fit a slightly larger waist size than 32 inch and can likely fit up to a 38 inch waist size as the waistband of the sweatpants stretch.
You use a drawstring to tighten the sweatpants around your waist.
I have a 32 inch waist and wearing medium sized sweatpants and there's still quite a bit of room in the waist for them to fit a larger waist size.
So if you have a 32 inch waist size then get the medium sized sweatpants or if you have larger than a 38 inch waist size then get a large size sweatpants.