Which carousel size to use
4:5 (1080×1350) takes up the most room in the feed, so it’s the usual pick. 1:1 is safest if you want every slide to look the same everywhere, and 3:4 matches the profile grid.
Every slide in a carousel shares the first slide’s shape, so they all come out the same size here.
Make the swipe worth it
Put something across the seam of the first two slides — a line, an arm, a word cut in half. It tells people there’s more to the right and gets the swipe.
Design the panorama for the grid, too
A seamless carousel is one wide image cut into equal slides. Plan it on a canvas exactly as wide as the slides together — five 4:5 slides make a 5400×1350 canvas — and keep important things away from the seams, where a face or a word would be split awkwardly.
Only the first slide shows on your profile grid and as the cover in most feeds, so it has to work on its own: put the hook, the face or the title there, and let the rest of the panorama reward the swipe.
Instagram compresses images, so export the panorama at full size and avoid tiny text. Everything here runs in your browser; the slides are named in order so you can upload them as one carousel.