Instagram carousel size
The whole carousel takes the ratio of the first slide.
Cropped in your browser · your image is never uploaded
1080 × 1350 or 1080 × 1080, but the important rule is that every slide has to be the same ratio. Instagram takes the ratio from the first slide and crops the rest to match.
How to use the instagram carousel size
Mixing ratios is the mistake that ruins carousels. If slide one is square and slide three is portrait, slide three gets cropped to square — usually through the middle of whatever it was showing. Decide the ratio before designing anything. Portrait is generally the better choice for the same reason it is in a normal post: more vertical space in the feed. Two design notes specific to carousels: leave a margin on the right of each slide, because the next slide peeks in slightly, and give people a reason to swipe on slide one, since the swipe rate is what determines whether the rest is seen at all.
Questions
The same ratio, yes. Instagram takes the ratio of the first slide and crops the others to match.