Depending on the type of system you are designing, you need to estimate its scale: how many users will access it every day, how much data you will store and process, etc.

You should be careful when making assumptions. Incorrect assumptions can dramatically change the design. You should rely on your product sense as well to make reasonable assumptions.

In the TinyURL example:

• It matters whether newly generated URLs must be accessible immediately
• It matters whether URL analytics must be updated in real-time