Where this number comes from: Instagram uses 1080 × 1920 (9:16) for stories and reels.
A story fills the whole phone screen, so anything that is not already 9:16 gets either letterboxed with blurred bars or cropped by Instagram on your behalf. Resizing first means you decide which parts survive rather than finding out afterwards.
9:16 is an unusually tall frame and almost no camera produces it. A normal 4:3 or 3:2 photo has to lose a lot of its width to fit, which is why the crop matters more here than at any other size: this tool covers the frame and keeps the middle, so a centred subject survives and a subject near an edge does not.
Keep anything that must be readable away from the top and bottom of the frame. The interface — profile name, timestamp, reply box — sits over those areas, and text placed there is covered on some phones and not others.
Questions
Is the story size the same as a reel?
Yes, both are 1080 × 1920 at 9:16. A cover image for a reel uses the same frame.
Why does my photo get cropped so much?
Because 9:16 is far taller than any normal photo. A 3:2 landscape shot has to lose roughly two thirds of its width to fill it. Crop deliberately first if the subject is off-centre.
Can I add the bars myself instead of cropping?
Not with this tool — it fills the frame. If you want the whole photo visible on a coloured background, place it on a 1080 × 1920 canvas in an editor and export that.