The ideal Instagram post sizes are: Square photos/videos (1080×1080 px, 1:1 ratio), Portrait (1080×1350 px, 4:5 ratio), and Landscape (1080×566 px, 16:9 ratio). Instagram Stories, Reels, and IGTV videos should be 1080×1920 px (9:16 ratio) for the best quality and full-screen experience.