What is the minimum size of a letter?

The correct answer identifies the minimum size of a letter as 5 x 3.5 inches. This measurement adheres to standard guidelines for letter size used in mailing and communication practices. Specifically, this size meets the requirements for letters that can be delivered via the postal system without incurring non-machinable surcharges.

These measurements are significant because they ensure that the letter meets the visual and practical norms for handling and sorting by postal services. Letters smaller than this specified size may not be recognized as standard mail and could lead to additional handling fees or delays in delivery. Thus, choosing 5 x 3.5 inches as the minimum size is crucial for maintaining efficiency in mail processing.