Choosing the right video size for Facebook ads is crucial to capture attention and engage viewers. Square videos (1:1 aspect ratio) are perfect for mobile devices, while vertical videos (9:16 aspect ratio) are ideal for feed posts and immersive experiences. Horizontal videos (16:9 aspect ratio) work well in full-screen video ads on desktops. Consider ad placement, target audience, and video content when selecting a size to optimize engagement. Optimize your ad with attention-grabbing moments, immersive experiences, and clear Call-to-Action (CTA)'s to increase conversions.