DL actually refers to a standard envelope size, normally used for folded letters and documents, but it is also used to define the size of a piece of paper that would fit in the envelope.

DL flyers are 98 x 210mm, about 1/3 of A4, and this is a great size for a handy price list or to advertise an event, as they fit into the slimline leaflet holders often found in hotels, supermarkets and tourist centres..