Every platform crops your picture to its own shape. Deciding that crop yourself is the difference between a clean post and one with someone's head cut off.
Feeds display a fixed aspect ratio and will crop anything that does not match. A wide landscape photo posted where a square is expected loses both ends, and the platform picks what to cut, not you.
Enormous resolution is wasted. Almost every platform re-encodes what you upload and serves something much smaller, so a 6000 pixel original gains you nothing except upload time. Around 1080 pixels on the long edge is the sensible ceiling.
Profile pictures are usually displayed as circles even when you upload a square. Leave a margin around the face, because the corners of your image will not be visible.
Platforms change their numbers regularly. Match the shape, keep the long edge around 1080 pixels, and you will be fine nearly everywhere.
Yes, resize a batch with the same settings.
No.