Any document larger than the two standard paper sizes (8.5" x 11 legal size,  8.5" x 11  letter size) is considered to be a large format document.
A few typical examples of large format documents includes:
Architectural blueprints
Engineering diagrams/drawings
Large non-standard book pages
Newspaper sheets
Maps
Schematics
The main reason for the "large format document" distinction is the need for special scanning and printing equipment when working with oversized documents.