A 'Stories' post is a story you tell about yourself. It should be informational, not pure promotion. For example:
Write about the new chef, not about the lunch special. (That's a Market or Event post.)
Share expert tips on skin care, rather than promoting a new skin cream you're selling. (That's a Market post.)
Short posts work as well as long ones, in terms of generating readership. Think hard about your headline and your photo -- these are how you'll grab a reader's attention in the DailyUV feed or on Facebook.