Your size chart is a part of your fashion brand ID
Did you ever tried on clothes you like in a shop then discover the size doesn't fit you at all? You are not that fat but you have to try on an XL or XXL? Whatever you try on, it's too long or too short? Well, personally I hate this and it's enough for me to get out of the shop and never buy from this brand again!
This comes from a bad size chart
What is a size chart? It's the list of the sizes you produce and sell to your clients.
That means there are several size charts? Yes, there are. The brand team decides what size chart they are working with. It's entirely their choice.
As a Fashion Designer, what size chart do I have to adopt? Don't copy other size charts before thinking about your clients. Are they on the large sizes or on the tiny size? Are they tall and fit or small and not-that-fit? if they are working out a lot, shoulders and arms are large (men) and shape is more rectangular (women). Or women have a more narrow waist and their shape is hourglass type?
Study your clients to build your own size chart. You are totally free to use your own size chart and to give each size a name you like. M, L, XL are industrial names, aren't they? Why not positive names like tiny, charming, gorgeous, generous?
Your average size must fit your average client. It's as simple as that but this very important detail is too often neglected. Make your client feel his(her) figure is enhanced by wearing your clothes. Don't make them ashamed of their body, it's the worst mistake you could do if you want to sell them any garment.