Designers, beware of letting your fit model self-diagnose her own size. It is a slippery slope towards a wonky, nonsense size range that I’ve seen too many designers fall into.
Yes, you do want to ask what size your fit model usually wears, but don’t rely solely on her answer to guide your size chart development. Use it as a data point and a check point for your brand’s own sizing.
It can be tempting to build your brand’s size chart around the self-diagnosed size and measurements of your fit model(s). Don’t do it. Every brand’s sizing is different (especially when you get into plus sizes) and so each person is self-diagnosing their size based off of the brands they shop from and their own perspective of their body.
Relying on your fit models to measure themselves is equally varied. The more people you add into the mix in more sizes, the more confusing your size chart can get if you rely solely on each of the self-diagnoses.
A better approach that balances individual feedback with consistent data, is to develop a draft size chart for your brand and then test that against each person’s assumed size and measurements. Measure each person yourself, if possible, and compare their measurements and self-diagnosed size to your size chart. Do they line up or do you see common differences?
You can certainly adjust your size chart based on fit model and customer feedback, but don’t start there. Start with a logical, consistent foundation and tweak as needed. Keep in mind the big picture of your full size range and don’t let any one individual’s self-diagnosed size sway your whole brand.