Different ad objectives recommend different sizes. If you'd like your ad to be eligble to show in all of the different formats, including desktop News Feed, mobile News Feed and the right column, then you should use the recommended image size for you objective which is stated below.
Please Note; If your image is larger or smaller than the dimensions specified in the Facebook Ads Guide, it will be automatically resized to fit an ad. Animated or flash images are not supported.
Carousel Format Ads are recommended to be 600 x 600 pixels regardless of your ad objective.
Recommended Image Sizes
Clicks to Website 1,200 x 628 pixels
Website Conversions 1,200 x 628 pixels
Page Post Engagement 1,200 x 900 pixels
Page Likes 1,200 x 444 pixels
App Installs 1,200 x 628 pixels
App Engagement 1,200 x 628 pixels
Local Awareness 1,200 x 628 pixels
Event Responses 1,200 x 444 pixels
Offer Claims 1,200 x 628 pixels
Video Views 1,200 x 675 pixels
Lead Generation 1,200 x 628 pixels
Want to know about ad objectives then read What Facebook Advertisising Objectives Can I Chose From
