The maximum size for a flat size mail piece is:

The maximum size for a flat size mail piece is defined by postal regulations to ensure mail can be processed efficiently. The correct dimensions for a flat size mail piece are specified to be up to 12 inches in height and 15 inches in length. This size allows for flat mail items, such as large envelopes and catalogs, to be handled properly through the postal system without hindering machine processing and delivery.

In this context, option B aligns with these established postal size limits, hence it is selected as the correct answer. Understanding these dimensions is crucial for anyone involved in mailing services, as it affects how packages and letters are classified and can influence shipping costs as well.