Quedar and quedarse. They have different meaning or uses. I can show how they differ but if you look in a dictionary you will see that quedar has many uses, too many to write down. Quedarse = To stay, me quedé en Madrid. Another use could be, me quedé sorprendido (surprised) me quedé dormido (I fell asleep and or I overslept. Quedar = He quedado con María para cenar (to arrange to meet) No me queda pan (I don't have any bread left. Queda poco tiempo para terminar el informe (I don't have much time to finish this report. A esa mujer le queda poco tiempo (This woman does not have long to live) As I am English, I hope that a native speaker can confirm this for you.

We use the verb "Quedar" with people and things staying in a place for a period of time. I explain your examples: De ese modelo sólo queda un par grande. (queda = hay [del verbo haber]*) * The verb Haber means in this case: There is sth real or figurative. Ex. I only have a big pair from that model. Quedar/Quedar(se) for people: Me quedé sentado toda la mañana. I sat all morning. [I was in a place for a long period of time] No me quedo solo, mis amigos están conmigo. (quedar = encontrarse, estar) I am not alone, my friends stay with me. No quiero quedarme aquí. I don't want to stay here. I hope this explain help you.

- Puedes quedarte ( to stay) en la casa - Quédate (to save) con la devuelta, del dinero. - Me quedé de encontrar con mi novio en el cine.