What boy size would be a medium in women's?

Honestly, I’d go to Walmart and try them on. It’s relatively easy to guess sizing when transferring from womens to mens, but boys sizes could range dramatically from clothing article to article, from brand to brand. Kids are always growing, so it’s hard for the manufacturers to produce exact sizing. You may be a size L in one pair of boys pants, an M in another, and an XXXL in another! Maybe you could try finding some second hand on an auction website, and ask the seller for the waist measurements, or even ask the seller if they would fit a woman. It’s not so much the waist, but the length too. Also remember womens pants are allowing for wider hips, even if you are a size medium in womans pants, you could end up being a large in MENS because men have slimmer hips. If you are a large in mens (just speculating here), you will most probably not even fit a child’s size. It’s the same for all women, we have wider hips and longer legs, making it very hard to fit children’s sizes waist wise and length wise.

Maybe you have a friend that could try them on for you if you can’t get to the store. Or, better still, you could ring Walmart and ask to speak to a woman, and ask her if she thinks they could fit a medium woman!