We give upper bounds on the principal curvatures of a maximal surface of nonpositive curvature in three-dimensional Anti-de Sitter space, which only depend on the width of the convex hull of the surface. Moreover, given a quasisymmetric homeomorphism φ, we study the relation between the width of the convex hull of the graph of φ, as a curve in the boundary of infinity of Anti-de Sitter space, and the cross-ratio norm of φ.As an application, we prove that if φ is a quasisymmetric homeomorphism of RP 1 with cross-ratio norm ||φ||, then ln K ≤ C||φ||, where K is the maximal dilatation of the minimal Lagrangian extension of φ to the hyperbolic plane.