A leaf functions as the paper used by the avout.
Leaves are the leaves of a genetically modified tree known as the page tree. Groves of these trees are placed inside a concent. In the fall, leaves are harvested by hand and then strung on lines to dry for several weeks. The leaves are then taken inside and stacked, with rocks being placed on top of the stacks in order to compress them. These stacks are left for 100 years before being taken out, cut into rectangles in cutting-frames, and used.
Typically around 10% of the harvested leaves are of the right texture and size to be bound into books. Other leaves may be suitable for other more routines uses.
Anathem refers to the high quality leaves as being used in "quarto-sized" books. On Earth this is a book approximately 23 cm x 30 cm (9" x 12"). This is likely the intended size of a leaf although "quarto" is also a term used in book binding for pages made by taking a single sheet and folding it twice, which would make each sheet approximately 46 cm x 60 cm (18" x 24").