Every Instagram photo size
Instagram uses one width, 1080 px, and three shapes for posts and Stories. Pick the shape first and the size follows.
Post (square)
1080 × 1080 px, 1:1 Feed post or carousel slide.
Post (portrait 4:5)
1080 × 1350 px, 4:5 The tallest shape the feed shows without cropping.
Story / Reel
1080 × 1920 px, 9:16 Full screen. Keep text away from the top and bottom edges, where the app draws its buttons.
Profile picture
320 × 320 px, 1:1 Shown as a circle.
One photo, three Instagram shapes
Most phone photos are 3:4 and most camera photos 2:3. Neither matches Instagram exactly, so something gets trimmed. The tool crops from the center: a 3:4 phone photo loses a thin band at the top and bottom to become 4:5, and a lot more to become square.
A Story is the opposite problem. A landscape photo has to lose most of its width to fill a 9:16 screen. If that cuts off the subject, post the square version to your Story instead and let Instagram place it on a background.
The tool warns you when a size is larger than your original. Instagram shows images 1080 px wide, so a smaller original looks soft once it is stretched to fill the screen.
From resized photo to a post that works
A correctly sized image is the start of a post, not the end. Write the caption with the first line doing the work, since only the start of it shows before "more", and decide what you want people to do: comment, save or send you a message.
If you ask people to comment a word to get something, plan how you will answer. On a post that does well, replying to every comment by hand takes hours. That is the part SociaHive automates.