Crop a video to 4:5, the portrait ratio for Instagram and Facebook feed posts. The standard export size is 1080x1350.
4:5 against the alternatives
A feed post can be square, 4:5, or landscape. 4:5 takes the most vertical space of the three, which is why it has displaced 1:1 as the default for feed video.
It stops short of 9:16. A full-height vertical clip belongs in Reels or Stories, not the feed, and posting one to the feed usually means it gets cut back to 4:5 anyway.
Crop or pad
Cropping a 1920x1080 frame to 4:5 keeps an 864x1080 box: 45% of the width. That is a milder cut than 9:16 but still enough to lose a second person at the edge of the shot.
Fit with bars keeps the full 1920x1080 picture and pads it into a 1536x1920 frame instead. The picture sits 1536x864 across the middle with 528 px of background above and below.
Starting from vertical footage
A 1080x1920 phone clip cropped to 4:5 keeps 1080x1350 and loses 570 px of height, which usually comes off the top and bottom of the subject. Drag the box up or down before exporting rather than accepting the centered default.