Every time I have read that with a group of students two interpretations arise with the phrase "extended full length." Some believe she is laying down on the couch resting and they believe that can fit with two women on opposite ends of the couch being buoyed up because this culture was lavish and that couch could have been seriously big. Wind was blowing through and these women were allowing it to blow their dresses high in the air (thus buoyed). After the wind died down is when Nick notices her extended full length. If she wasn't laying down, she was maybe not sitting like a lady, but slouched with her feet stretched to the floor.
Either of these interpretations could cause Nick notice because the twenties was a time of change for women and Jordan right here embodies that. If she is indeed slouched, or stretched out, or immovable in the presence of a guest to their home and has her chin up, she's being rude! As our first introduction to her, she is indicative of the flapper attitude and maintains that throughout the story. But for a man to watch this was probably counter-cultural to the way he was raised to see a woman respond upon the entry of a guest. He was likely raised to see a set of pleasantries exchanged.
I also think Jordan is probably attractive and Nick is mentally noting that.
This scene you are describing happens in Chapter 1.
I believe that Nick notices Jordan for how still she seems. He notices that she and Daisy are the only perfectly still things in the room, but he seems to notice Jordan more. She is so poised and still that it looks to him like she is balancing something on her nose.
Her calm and her stillness sort of intimidates him. It makes him want to apologize for disturbing her.
So I guess I would say that he notices her because she seems so poised and controlled and calm.