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.
In the feed, no. 4:5 and 1.91:1 both display in full. On your profile grid, yes, everything is cropped to a square.
Instagram re-encodes everything. Upload at exactly 1080 wide rather than larger, and their downscaler is taken out of the equation.
Instagram caps display around 1080 pixels wide. Uploading larger does not improve it and often makes it worse.
Yes, at 1080 × 566. It takes less than half the feed height of a portrait, so it works far less hard.