The dimensions of an Instagram post depend on the orientation of the image and the tool used to upload it. However, the most common Instagram post size is 1080 pixels by 1080 pixels, with a 1:1 square aspect ratio. Instagram keeps the photo at its original size as long as its height is between 566 and 1350 pixels, and the width is between 320 and 1080 pixels. If the photo is less than 320 pixels wide, it will be stretched, and if it is more than 1080 pixels wide, it will be shrunk down. For portrait photos, the aspect ratio is 4:5, and the recommended size is 1080 pixels by 1350 pixels. For landscape photos, the aspect ratio is 1.91:1, and the recommended size is 1080 pixels by 608 pixels.

In summary, the ideal dimensions for an Instagram post are 1080 pixels by 1080 pixels for square photos, 1080 pixels by 1350 pixels for portrait photos, and 1080 pixels by 608 pixels for landscape photos.