Large-format generally refers to any wine or spirits sold in a bottle size larger than the standard 700mL (spirits) or 750mL (wine). Common sizes include a value 1L size, also popular for limited-edition whisky expressions, and 1.5L Magnum that holds twice as much wine as a standard bottle.
The Jeroboam (3L) and Methuselah (6L) are also popular, and even larger sizes exist - these are similarly named for Biblical figures.