Unless you are a big brand with bottomless pockets you won't be able to create content at it's perfect size for every different social media platform. That's why it's important to create your video at a certain size so it plays well across the different platforms. When deciding on your size you should think about what platforms your content is going to.
1080 x 1080 pixels (1:1) is the sweet spot for most projects which looks good across Facebook, Instagram, Twitter but on YouTube it is going to be pillarboxed which means there will be black bars on the left and right of the video.
1920 x 1080 (16:9) which is the preferred size for YouTube but also works well on Facebook, Instagram and Twitter. This is the most common video size you will see out there in the world at the moment.
1080 x 1920 (9:16) also known as vertical video works great for content that is viewed on mobile specifically Snapchat, Facebook and Instagram stories.
It does get a lot more complicated then that when you dig deeper and it's always changing which is why we always check in on the always updated guide on Sprout Social.