This dissertation critically reassesses Jean Dubuffet's collection of Art Brut in light of both the manifold contradictions of Dubuffet's mythologizing texts, and the materiality and dimensions of its constituent artworks. Dubuffet coined the term Art Brut in 1945 to designate artworks made by untrained and non-professional artists. From 1947 to 1976, with the help of the Compagnie de l'Art Brut, he created an important collection of thousands of artworks - the Collection of Art Brut - gleaned from psychiatric hospitals, rural settings, and jails. For Dubuffet, the works questioned the taste and values of his period, challenging both artistic categories and conventional display practices. This thesis shows how the materials, methods of creation, and dimensions of the works collected by Dubuffet and his companions helped construct a powerful artistic myth during the middle of the twentieth century. The dissertation first analyses ways in which "poor" materials and extreme formats (ranging from very small to very big) helped support the phantasnatic idea of an Art Brut that is "humble", " natural", and "raw". The thesis then reinscribes the project of Art Brut into the history of European avant-gardes in order to show the project's specificity and political consequences. Finally, the thesis demonstrates ways in which the formats of Dubuffet's collected works led to a reform of museum space.