I have a secondhand Valencia full size classical guitar and after using it for 30-40 minutes I tend to get a stiff right shoulder and some backache.

I think I should get a guitar that is more suitable for my size

My question is which guitar size is more suitable for me as I am a short (5'7") person with relatively short upper arms

My current guitar is 4" deep at the body and 14.5" wide at the widest point.

any ideas?