World Building: Where Should You Start?

I’d wager that every GM of at least a few months of experience has either considered “world building” or done it. It seems like a massive, maybe daunting challenge, and it can be. Where, then, should you start? Herein we begin our extended series on world building:

• What size should you make your new world?
• And by "world," what do you mean?
• How to think about which system to use
• Where to find inspiration
• When to start "building