The proton beam accelerated by the interaction of laser with plasma has practical applications in radiography of dense plasma, fast ignition in inertial confinement fusion, and cancer treatment. The application domain is determined by the characteristic of the proton beams, which is affected by a lot parameters. In order to investigate the effect of the initial size of the proton layer, the two-dimensional Particle-In-Cell (2D-PIC) code Flips2D is used. The total energy of proton beam vs. time is studied, and the relation between the duration of acceleration and the period of laser pulse is obtained. The effects of the proton layer initial width and thickness on the divergence angle and the energy spectrum of the proton beam are investigated. The relation between the proton beam characteristics and proton layer initial size is obtained.