From a futuristic viewpoint, one would argue that the paradigm of good audience size has shifted significantly due to advances in hyper-targetted ads and increasingly intelligent algorithms. Nonetheless, it’s not really about the size anymore but rather about the quality and relevance of the audience.

Keep in mind that large audience sizes might get you more impressions but not necessarily quality engagement or conversions. Conversely, an extremely niche audience might result in higher engagement but limited overall reach.

A step-by-step approach could be:

Step 1: Define your campaign goals. Is it brand awareness, traffic, conversions, app installs, etc.?

Step 2: Identify your target audience based on your campaign goals. This could include demographic information, interests, behaviours, etc.

Step 3: Use Facebook’s audience insights tool for in-depth analysis of your potential audience. It will give you a rough estimate of the audience size that you can reach.

Step 4: Start with a broad audience and let the machine learning tools in Facebook’s ad system find the highest-performing segments. Then refine and iterate based on performance.

Step 5: Continuously monitor and adjust. Remember that audience size is not static. It’s a living, breathing thing that can and will change over time.

This is all, of course, assuming Facebook Ads continue to exist and function similarly in the future, which could be a wild assumption given the rapid pace of tech evolution!