My 2 cents, it’s usually a good idea to have the size be at least 2x what you expect most people to view it as, so it looks nicer on modern phone screens that are high resolution. The Micro.blog timeline currently defaults to scaling down to 1000 pixels wide. The client apps usually resize to 1800 pixels before uploading, then add a width tag of 600 making it roughly 3x the display size, but it’s not an exact science with so many different screen sizes.