The word Quarto is used to describe an item which is too large to fit on the ordinary shelves. Where Library Search (the catalogue) shows Quarto against an item, it means that the item is shelved in the separate Oversize section of the Library.
Library Search will tell you the shelfmark and which building and floor to look on. Quarto shelves can be identified by the labels on the shelf-ends. Once you have found the quarto shelves, find your item in the usual way by looking for the shelfmark.