Instagram post size
The one to use. It takes the most vertical space in the feed.
Cropped in your browser · your image is never uploaded
Post at 1080 × 1350, the 4:5 portrait. All three shapes are accepted, but portrait occupies the most vertical space as someone scrolls, and on a phone that is the only thing that matters.
How to use the instagram post size
The catch nobody mentions is that your profile grid crops every post to a square, regardless of the shape you posted. So a 4:5 portrait loses the top and bottom 12.5 percent on the grid while showing in full in the feed — meaning anything that must survive both, such as text or a face near an edge, has to sit inside the central square. Instagram also re-compresses uploads: sending a 1080-wide image gets you close to what you uploaded, while sending a 4000-wide one gets it downscaled by their encoder, which is generally worse than downscaling it yourself first.
Questions
1080 × 1350, the 4:5 portrait. It uses the most screen height in the feed, which is the whole game on a phone.