Here is a quick overview of creative best practices 😎
Single Image Ads & Carousels:
1080 x 1080 px
Instagram and Facebook Stories:
1080 x 1920 px
Square (1:1) up to 60 seconds (but better if less than 15secs). If possible with sound and captions. 4GB max.
Check out Facebooks Ad Guide for a more detailed explanation.