Another useful guide from the Social Media Examiner.
How Lead Generation Ads Work
Facebook and Instagram lead generation ads are designed to deliver email addresses directly to your database. Unlike other ad types, lead ads include a contact form for people to fill out if they’re interested in your product or service.
Facebook pre-populates the lead form with user information they already have. Because the ad does most of the work of filling in the data, users are more likely to complete the form, delivering you a new lead.
After leads are captured, Facebook lets you download them manually or you can connect to a CRM such as MailChimp or Salesforce.
More useful advice from the folks at Buffer:
In general, here are the best sizes for sharing images on social media.
Facebook – 1,200 x 628
Twitter – 1,024 x 576
Instagram – 1,080 x 1,080
LinkedIn – 552 x 368
Pinterest – 600 x 900
Google+ – 800 x 320
And here is "the Essential Social Media Design & Sizing Cheat Sheet" from omnicore: