Three ratios cover almost everything. Square for feed posts, which displays acceptably nearly everywhere. Vertical at a tall ratio for stories, reels, and short video, which is now the dominant format for anything that moves. And a wide landscape ratio for link previews and cover images. If you produce a version at each of those, you can post almost anywhere without redesigning.
Profile images deserve specific care because they are cropped to a circle on most platforms while being uploaded as a square. Anything positioned near the corners disappears. A full logo with a wordmark usually becomes illegible at the size these display, which is why the icon or mark alone, without the words, is the correct choice for a profile picture. If your designer supplied only a horizontal logo, that is a gap worth filling.
Design for the smallest size the image will appear at rather than the largest. A graphic that reads beautifully on your monitor is frequently unreadable in a phone feed at a fraction of that size. The practical test takes ten seconds: view it on your phone at actual size before posting, and if you cannot read the text without zooming, neither can anybody else.
Keep the source files rather than only the exports. Templates in whichever design tool you use, sized to those three ratios, mean the next graphic takes minutes rather than starting over. That template library is worth more than knowing any particular set of dimensions, which change whenever a platform updates its layout.
Build a small template set rather than remembering dimensions, because the platforms change their layouts and the templates outlive the specifications. One file per ratio, with your colours and typefaces already applied, means every future graphic starts from something correct rather than being rebuilt. That library takes an afternoon to establish and it is the thing that makes consistent output possible when you are producing something quickly.
Check how images appear in the contexts you do not control, particularly link previews. When somebody shares your page, the image pulled is whichever one you specified in the page settings, and if none was specified the platform picks something arbitrary, frequently a logo fragment or a stock photograph from partway down the page. Setting that deliberately for your main pages takes minutes and determines what your business looks like every time somebody shares it.