Best ad size for Facebook and its partners?

I'm trying to set up a Facebook ad for a client and I'm a little confused about the proper size. Most of the time, we simply design one 1200px x 628px ad and use Facebook's Automatic Placement to place the ad in various platforms, such as Facebook Feeds, the Facebook Right Column,  Instagram Feeds, and the Audience Network. As I'm sure you know, most of these platforms require different ad sizes and aspect ratios. So here are my questions:

1. To accommodate the various ad sizes, I'm assuming  Facebook proportionally scales the 1200px x 628px ad to fit each respective platform. If that's the case, wouldn't the ad performance suffer because it may not be maximizing the image space available for that specific platform.
2. Is there a best, general purpose Facebook ad size that will accommodate all of the ad placements mentioned?
3. If not, is it best to design ads for specific placements?
4. Where can I find the best, most up-to-date social media cheat sheet for ad sizes?

Thanks in advance for your help.