One size that works in the Facebook feed and on Instagram.
Portrait for the feed
Most people scroll on a phone held upright. A 4:5 portrait post is taller than a square or a landscape picture of the same width, so it takes up more of the screen as people scroll past.
One post for Facebook and Instagram
Melon Cut makes Facebook feed posts with the same 4:5 post it makes for Instagram, so one 1080 × 1350 file can go to both. Keeping content inside the safe area means Instagram's profile grid doesn't cut it either.
Making Facebook posts in Melon Cut
Choose Social post, describe the post and add your brand's @username. Check for social media then looks at the size, the shape and the content, using rules summarised from Meta's official policy and ad-spec pages.