With so many different common paper sizes around, it is hard to keep track of them. Though it's smaller than the standard letter-sized paper, a DL-sized paper has many diverse uses.

A DL-sized paper measures 99mm x 210mm. Therefore it's the equivalent of an A4 (standard US letter-sized) paper folded into thirds. It's also equivalent to an A5 piece of paper folded lengthwise in half. (Resource 2)

A DL envelope, however, is typically is 110mm x 220mm or 4 5/16 in. x 8 1/16 in. Therefore, it is often thought of as the standard "business envelope" since it can easily hold a standard U.S. business letter(A4-sized paper) folded into thirds. (Resource 1)

The DL size usually refers to an envelope. However, DL-sized paper may be used for common announcemets such as invitations, letter drop boxes, wedding announcements, or greeting cards meant to hold money. (Resource 3)