Most fashion returns are about fit, not taste. A customer who picks the right size keeps the garment; a customer who guesses sends it back and rarely reorders.
What an advisor does
It asks for a few measurements, compares them with the garment's size chart and recommends a size with a confidence note. That is it. No magic, just fewer guesses.
Where to place it
Next to the size selector, before the add-to-cart button. If it appears after the decision, it is a warning; before, it is help.
Feed it real charts
The advisor is only as accurate as the chart behind it. Measure your garments flat and enter the numbers; do not copy a supplier's generic table.
Measure the effect
Track return rate on items where the advisor was used versus not. Even a few points of difference pay for the effort many times over.