Girls, I need help understanding clothes sizes?

therealadamreed44
I want to get my girlfriend a two piece crop top jacket and matching skirt set for her birthday. The problem I'm having is according to the shops size guide, my girlfriend is a medium in her bust measurement and a small in her hips and waist measurements. But as it's a two piece set, it will only allow me to order it in one size, so which size do I order for my girlfriend?
Girls, I need help understanding clothes sizes?
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