A carousel is a sequence, so the transition between images matters as much as the first slide. Decide whether your set is mainly portraits, wide photographs, or designed graphics before exporting. This page starts at 1080×1350 pixels, our portrait setting. It gives a consistent working frame, but you can choose a square or landscape preset if that better suits the material.
Do not assume the same centre crop works for every source. One photo may need room above a face, another may need the entire product base, and a third may contain a sign near an edge. Review each image in the queue. If keeping the whole composition is essential, prepare a finished canvas in a separate editor rather than letting a crop remove important content.
Instagram's composer can offer different framing controls across accounts and versions. Check the actual publishing preview for the whole set. A consistent export makes preparation easier, but it does not guarantee how a platform will present mixed shapes. Avoid relying on an account feature you have not checked on the device you will use to post.