I find that harness fitting, particularly waist harnesses, is much more dependent on your shape than your waist size.
I'm a 34" waist, but 6'2" and 68kgs. There is no way I could wear a waist harness designed for a 34" waist, I would swim in it.
Small Airstyler and XS Pro-limit FX are the two waist harnesses I have found most comfortable when trying them on, however I ended up getting an Airstyler because they are more colourful

Maybe try a small, or even extra small, North Airstyler, the XS is tiny, the first adult harness I have every tried that I couldn't fit. The FX comes in a girls version too I think?