Poetry: We like poems that are comprehensible to most readers. We like to give each poem its own page, which also means the poem should be substantial enough to look good on a page.
Short fiction: 600-3,000 words.
Memoir: An interesting, 100%-true episode from your life. 1,200-2,400 words. The emphasis is on episode, as distinct from a whole life story.
Pensées: A short essay on some specific topic you've given thought to. 1,200-2,000 words.