In this work, we present the influence of dimensional parameters on dark current and photocurrent of the metal-semiconductor-metal photodetector (MSM). MSM photodetectors of different sizes have been fabricated on GaAs (NID). The active area of MSM samples varies between [email protected]^2 and [email protected]^2 with equal electrodes spacing and finger widths (l=D) varying between 0.2 and [email protected] The I(V) characterization in inverse and direct polarization in darkness shows good symmetry of curves, which shows the good performance of components and successful fulfillment of the Schottky contacts. The application of laser fiber of incident light power of 16mW at wavelength of 850nm for the illumination of the MSM photodetectors showed the evolution of the photocurrent ranging from 0.75 to 1.81mA, respectively, for 1 to [email protected] electrodes spacing at 3V and active area [email protected]^2. We showed also that variation ranging from 0.45 to 2.5mA, respectively, for [email protected]^2 to [email protected]^2 at 3V and [email protected] electrodes spacing. The resistance of MSM photodetectors obtained evolved proportionally to the electrodes spacing ([email protected] for [email protected] and [email protected] for [email protected] with [email protected]^2) and inversely proportional to the surface area ([email protected] for [email protected]^2, and [email protected] for [email protected]^2 with [email protected] inter electrodes spacing).