The issue isn't what size you post, it's what size someone sees. That isn't determined by a resized or not resized file. It is determined by your monitor and the operating system that presents an image at the largest size that the screen will accommodate. There's nothing any of us can do about that.
Because the monitor displays the horizontal dimension to fill the screen a vertical sized to 1024 will show as a 2 mp image on a 1080p screen. Rotate that same image to portrait mode and the display will show an image that looks 50% larger.
An 8k screen displays any image at the equivalent of 33mp.
A 720p screen displays at .9mp.
Post a picture resized to 1024 on the long side and it will be presented as 1280 at the smallest size. Post an image that is 5,665 on the long size and it will still be seen as 1280. The only caveat here is that if an image is only 300 p on the long size the image at 1280 may appear to be pixelated. Larger images are downsized to fit the screen.
The largest display currently available is 8k. That will present a 33mp image faithfully. 4k will present between 8mp and 4mp depending on the version of 4k you have.
It isn't the size you post, it's the size your monitor shows you. A small monitor will not make a big image look better but a huge monitor can make a small image look worse. There's nothing any of us can do about that.
*disclaimer: This presentation is oversimplified. Please let us not go further into the weeds to minutia.
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