ssize_t is used for functions whose return value could either be a valid size, or a negative value to indicate an error.
It is guaranteed to be able to store values at least in the range [-1, SSIZE_MAX] (SSIZE_MAX is system-dependent).
So you should use size_t whenever you mean to return a size in bytes, and ssize_t whenever you would return either a size in bytes or a (negative) error value.
See:
http://pubs.opengroup.org/onlinepubs/007908775/xsh/systypes.h.html