The short answer, then what it means for your logo and text.
Square or portrait?
Square posts (1:1) still work, but a 4:5 portrait is 25% taller at the same width, so it gets more of the screen as people scroll.
Making posts at the right size
Melon Cut Design's Social post format makes every picture at exactly 1080 × 1350, keeps important content inside the safe area, and can check a finished post before you publish it.