It'd help if we had a little more pertinent info.

Which guitar specifically are you referring to? Electric or acoustic, size if acoustic e.g. parlour or dreadnought.

Are you unsually tiny of build and stature or?

Correct posture is an ideal model rather than set in stone. The key is to adapt the basic principles of posture optimised to your individual situation. i.e. Individuals gave different length arms etc. Let your body guide you. You'll know what feels comfortable.