"Her back is large."
Translation:Su espalda es grande.
I used "su espalda" here because I guessed Duo would be dumb about the number issue here, but I'm pretty sure it should be "sus espaldas", if we want to talk about hir back, rather than just her shoulder. Espalda literally means something like the area around the shoulder blade; it's related to the same root from which we get "scapula". So, for instance, "volver la espalda a" is the expression "turn a cold shoulder to", while, "de espaldas a algo" is "with one's back to something". (In order for something to be behind your back, it needs to be behind both your shoulders.)
Shoulder blades are not on your shoulders, rather they cover the upper part of the human back and attach to the shoulder.
Do the words 'largo' or 'larga' exist in Spanish to describe something large in certain cases?