We propose a theoretical model for an optimized fiber structure for use in anti-Stokes laser cooling of solids. The sample is an optical fiber fabricated from a fluorozirconate glass ZBLANP with a core doped with Yb(3+) ions. The diameter of the fiber core is optimized to achieve the largest temperature change in the sample. It is shown that for each value of the pump power there is an optimized diameter of the fiber core, which permits the largest drop in the temperature of the sample.