I use this cheat-sheet: https://sproutsocial.com/insights/social-media-image-sizes-guide/
Bottom line: The image will be displayed at 1080 pixels wide and up to 1350 pixels tall, depending on the aspect ratio. So that is what you'd input for resizing. Of course it should be JPEG sRGB; the quality slider you can experiment with, but I doubt you'll see a difference in practice on most photos between 70, 80, 90, and 100.
https://www.reddit.com/r/photography/wiki/technical#wiki_how_do_i_shoot_in_low_light.3F