Giving an instrument from a relative or friend a second life might save you some money, when it does not have the appropriate size it will lead to many frustrations along the learning path or worse: an abrupt stop to guitar lessons.
Having the correct guitar size is crucial in developing a proper posture and playing technique in order to avoid injuries. Most nylon string guitar brands (Yamaha, Cordoba, Alhambra, …) offer several different guitar size models.
A meet up at a local music store to have the student ‘feel’ the comfort and difference of a ‘right-sized guitar’ is usually convincing enough.
As a guitar teacher, I personally have all guitar sizes at my home studio to experiment with when I feel a student is ready for an upgrade in guitar size.
TIP: Students often pass on instruments, so perhaps ask your local guitar teacher if any student has (a smaller) one for sale.
A ‘General’ Guideline on Guitar Size