The Instagram Portrait Post size (1080×1350px)
At 1080x1350 pixels the 4:5 portrait is the tallest still image Instagram permits in the main feed, and that extra height is valuable real estate. A vertical post occupies more of the screen as someone scrolls, so it naturally commands more attention than a square and gives you more room for products, faces, or vertical scenes.
This ratio is popular with creators and brands because it stops the thumb-scroll. Where a square shows the next post peeking below it, a 4:5 portrait pushes competing content off-screen for a moment longer. That is why so many high-engagement feed posts and carousels are built in portrait rather than square.
The catch is the profile grid: your beautiful tall image is cropped back to a 1:1 square in the grid preview, so keep the core subject in the vertical center. Avoid stacking essential text at the very top or bottom, since those bands are the first to be lost in the grid thumbnail and can collide with the username and action buttons in the feed. Design tall, but keep the message centered.