A size advisor only helps if shoppers finish it. Each additional question costs some of them, so every question should earn its place by improving the recommendation.
Start with the basics
Height, weight, age, fit preference and a couple of simple body-shape questions give a solid starting point for most clothing. Use sensible defaults, so a typical shopper only changes one or two answers.
Add anchors
"Which brand and size do you usually wear?" is one of the most informative questions you can ask, because it ties the shopper to a real garment. Make it optional but visible.
Ask by category
Only ask what matters for the product. Foot width matters for shoes, not for t-shirts. Keeping the form short for each category is the easiest way to keep completion rates up.