Flexible rolling process (FRP) is a rapid and effective three-dimensional (3D) surface forming process. Forming tool is a pair of bendable forming roll sets controlled by controlling units. In this process, the transversal bending deformation is obtained by the shape of forming rolls. The longitude bending deformation is obtained by the uneven longitude elongation of sheet metal which is due to the uneven thickness thinning. The sheet metal is continuously bent in the transverse and longitudinal directions at the same time. Therefore, the 3D surface part is formed. The deformation process is cumulative, so the size of the sheet metal has a significant influence on the forming effect. In this paper, taking the convex part as an example, the forming effect of different length and width of sheet metal is discussed through numerical simulation in FRP. The results demonstrate that FRP is more sensitive to the width of the sheet metal and is almost unaffected by the length of the sheet metal. In the case of a constant width, the utilization of the sheet metal can be increased by increasing the length of the sheet metal in a certain extent.