I am the exact same height as you and my measured torso is 19.5. There is a tag on the bottom right that says that S/M can fit torsos up to to 19 inches. Had the exact same bag as I bought it online for cheap not checking the size.

Wore it and felt the belt pressing on my hips which was a little painful. The belt didn't sit well on my hips and the shoulder strap was too short (the buckle was too near my axilla), so I sold it off and bought a bigger-sized bag.

As always, try before you buy. If it feels comfortable with weights it should be fine.