What is the minimum size of a postcard compared to?

The minimum size of a postcard is compared to a letter, as postcards are classified within the mail types by size and format. A postcard must meet certain dimensional requirements to qualify for mailing at postcard rates, which typically include being rectangular and not exceeding specific minimum and maximum lengths, widths, and thickness. The dimensions of a letter serve as a standard for understanding the size scale in mailing categories.

In contrast, the other options refer to different categories of mail that have their own size and weight specifications, but these categories are not the benchmark for the minimum size of a postcard.