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
In Cutlin, the Portrait 4:5 size is tagged both Instagram post and Facebook post, 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 Cutlin
Choose Portrait 4:5 in Configuration, describe the post and add your brand's @username. To remake a picture at this size with content inside the safe area, use Generate for social media in its menu. 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.