Optical fiber sensors based on the light backscattered by particles are widely used to measure the volume fraction of suspensions. But all the users know that particle size influences the sensor response. In this paper, this particle size effect is studied for one sensor geometry (a central emitting fiber and two crowns of receiving fibers). An electronic device gives an output voltage in terms of the ratio of light intensities collected by each crown. Experimental results obtained with several suspensions, spherical glass beads and silica particles suspended in water, are scaled on a single curve in terms of the C/d(3,2) ratio. C is the volume fraction and d(3,2) the Sauter mean diameter, also called volume surface diameter, characteristic of size particle distribution. This single curve is validated for various particle size distributions by calculating the sensor response with Monte Carlo simulation when the particles are spherical, weakly absorbent and their size greater than the light wavelength. So, the validity range of this curve is: 0.1 less than or equal to C less than or equal to 0.4 with 7 mum < d(3,2) < 25 mum. A possible application of this result is the study of size segregation during the sedimentation of a concentrated suspension, if the concentration is measured by other means (for instance by an acoustic device).