If your Reel ever showed up cropped or with black bars top and bottom, you've run into Instagram's spec requirements. Here's what Instagram's own Help Center says.
Aspect ratio and resolution: Instagram accepts anything from 1.91:1 to 9:16, but the recommended ratio is vertical 9:16. The minimum resolution is 720px, with 1080×1920 (vertical full HD) recommended. If you upload a horizontally shot video as-is, you'll get black bars or cropped sides — shooting vertically from the start gives the cleanest result.
Length: Recording and uploading support up to 20 minutes. But keep in mind that Reels over 3 minutes aren't recommended to non-followers — if you want your Reel to reach a wide audience, keep it under 3 minutes. Paid "Boosted Reels" have their own stricter rule: 90 seconds or less, and full-screen 9:16 is required.
Frame rate: At least 30fps is recommended.
File format: MP4 and MOV are the common choices.
If your footage is too large to upload quickly, use Compress Video to set the resolution (1080p) and enter a target file size.