This is for things like IGTV, IG Reel, and Tik Tok. On IG the best frame ratios are 4:5 (1080x1350) and 1:1 (1080x1080), but when posting IGTV, IG Reels, and TikToks, it's best to post in 9:16 (1080x1920).
An excellent way to check and see your ratio is to take the content into Inshot, go to the Canvas function, and select ratio. That will show you all the sizes and allow you to adjust.
DISCLAIMER: Make sure that your content takes up the whole frame when you resize; you don't want any colored or blurry bars around the content.