General Suggestions:
Always design square photos to 1080×1080 and shrink them to the appropriate size.
Phone sized pictures/videos (instagram story, tiktok video) are 1080×1920.
Social Media
Instagram Post: 1080×1080
Instagram Story: 1080×1920
TikTok Profile Picture: 200×200
TikTok Video: 1080×1920
Facebook Banner: 820×312
Facebook Profile Picture: 1080×1080
Twitter Profile Picture: 400×400
Gaming
Discord Profile Picture: 120×120
Xbox Gamerpic: 1080×1080
Playstation Avatar: 440×440
Steam Pic: 184×184