Resize a video to 1080x1080, the square frame that behaves the same in every feed.
What square buys
A square reads identically whether the phone is upright or sideways, and every platform accepts it without special handling. On a desktop feed it stays a reasonable size, where a 9:16 clip becomes a narrow strip.
The cost is vertical presence. On a phone, 4:5 occupies more of the screen and pushes competing posts further down. Square is the safer shape, not the most prominent one.
Coming from 1920x1080
The largest square inside a 16:9 frame is 1080x1080, so crop to fill cuts 420 pixels from each side and keeps 56% of the width. That is mild compared with a 9:16 crop, which keeps 32%.
To keep all 1920 columns, pad instead of cutting in the video cropper: the picture sits as a 1080x608 band across the middle with 236 pixels of background above and below. On a square canvas those bars are shallow enough that a blurred backdrop rarely reads as padding.
Where square is the wrong choice
Reels, Shorts, TikTok, and Stories are full-screen 9:16 surfaces. A square clip is accepted, but the platform pads it and you do not control how. Export 1080x1920 for those, and keep the square for feed posts.