Since my native language is Spanisch, (which I've been teaching in Germany for over 20 years now) I understand well the differences between SER und ESTAR. Yes, I get the meaning, but I would say it is very unusual to use ESTAR here. It's an unlucky example for people still learning the differences between SER & ESTAR, because, without context, even native speakers will first think someone is describing the skirt (a quality) and that's what SER is for. The idea that in this case, it's about a state (something unusual, just this time) isn't clear at all. Perhaps if you add "Hoje," it would be clearer, and those not having the 2 forms of "to be" in their native languages would be less confused.... just an observation.
- A saia da menina é comprida = The girl's skirt is long.
- A saia da menina está comprida = The girl's skirt is longer than it should be.
The verb "estar" usually express a "changeable" or a temporary state, instead of a state of being. It doesn't express an inherent trait of something, and that's why you say "eu estou triste" / "I'm sad" (at the moment), and not "eu sou triste" / "I'm sad" (sad is a part of my personality).