Learn Hardcore Hebrew: המוכרת אומרת שהמידה הזאת לא בחנות, אבל אפשר למצוא אותה באתר. - The saleswoman says that this size is not in the store, but it can be found on the website.

It is feminine because the base noun is מוכרת, the feminine form of מוכר (salesman / seller). The ה־ at the beginning is the definite article, so:

מוכרת = a saleswoman
המוכרת = the saleswoman

In context, this usually means the store clerk or the sales assistant.

Why is the verb אומרת and not אומר?

Because the subject המוכרת is feminine singular.

In Hebrew present tense, verbs agree with the subject in gender and number:

הוא אומר = he says
היא אומרת = she says

So with המוכרת you need אומרת.

What does the ש in שהמידה mean?