X and Twitter post image size: 1200 by 675

The width is chosen for the display rather than for the file. A timeline column on a desktop browser is typically around 600 CSS pixels wide, so 1200 device pixels gives a clean two to one match on a high density display without wasting bandwidth on detail nobody will resolve. Going much larger buys nothing visible.

The height follows from the ratio. Dividing 1200 by 675 gives exactly 1.7778, which is 16:9 after removing the greatest common divisor of 75. That makes it directly interchangeable with a video still or a 1920 by 1080 export scaled down, which is why it is the easiest social size to feed from an existing library.

The timeline does not always show the whole image. A single image in a preview can be cropped to a shorter shape, and a multi image post crops harder still, so the parts of the frame that matter should sit in the middle band rather than at the top or bottom. Opening the image shows it whole; the preview is what most people see.

The social post generator composes a Twitter post at 1600 by 900, which is the same 16:9 shape at a higher resolution, so an asset from it can be used here directly or scaled down. The cropper's 16:9 ratio gets an arbitrary source to the right shape first.