What sample size is: The number of visitors who see—and are part of—the test on your website.
Why sample size matters: When your test page comes with a high risk (for example, if you’re paying to drive traffic to this page or you cannot afford to lose conversions), you must be careful to limit your sample size to the minimum amount of visitors. This means just enough users to help you reach confidence in your data in a timely fashion. Otherwise, you may risk having a test negatively affect conversions, and result in unaffordable losses.
The bottom line: Devote time and resources to finding the “comfort zone” for your sample size in each test and limit all incoming traffic to suitably fit your budget.