Three sizes, and what each is for
The square is the safe one. It is what a profile grid shows, it survives being cropped to a circle for a display picture elsewhere, and nothing at the edges is at risk. The tall post at 1080 by 1350 is the greedy one: four by five is the tallest proportion a feed post is allowed, so it occupies more of the screen than a square does as somebody scrolls past. The story at 1080 by 1920 is the whole screen and nothing else.
| Shape | Pixels | Where it goes |
|---|---|---|
| Square | 1080 × 1080 | Feed post, profile grid, display picture |
| Tall post | 1080 × 1350 | Feed post that fills more of the screen |
| Story | 1080 × 1920 | Stories, reels covers, status updates |
Keep the edges clear on a story
A story is full-screen, and the top and bottom of that screen carry the app’s own furniture — a profile name and a progress bar at the top, a reply box at the bottom. Roughly 250 pixels at each end are covered on most phones. A collage that runs a slot edge to edge there will have that slot partly obscured, which is not a crop and cannot be fixed after posting.
The cheap fix is the outer edge slider. Push it up to forty and the whole arrangement pulls in from the frame, which reads as a deliberate border and keeps every slot clear of the furniture. The layouts with a full-width band at top or bottom are the ones to watch; the even grids rarely have the problem.
Switching shape does not cost you anything
Changing size re-lays the collage through the arrangement that was drawn for that shape rather than stretching the square one, and your photos stay where they are with their framing intact. So the sensible order of work is to fill the layout first and choose the size afterwards, trying all three before you save — it takes three taps and the difference between them is usually larger than you expect.
Everything saves at 1080 pixels on the short edge, which is the width Instagram compresses down to anyway. Sending something larger does not buy detail; it just means the compression happens on their side rather than on yours.