The format of a painting is its shape, which will almost invariably have been dictated by its function, and by the intended physical context. Duccio's triptych, 'The Virgin and Child with Saints', is a small portable work, intended for private devotion, while Tiepolo's painting of 'An Allegory with Venus and Time' was intended for a particular palace ceiling.
Paintings of landscape format, so called after the type of subject they depict, are wider than they are high; paintings of portrait format are higher than they are wide.
Paintings with a round format are called tondos.
