We have derived a model of the Kuiper belt luminosity function exhibited by a broken power-law size distribution .
This model allows direct comparison of the observed luminosity function to the underlying size distribution .
We discuss the importance of the radial distribution model in determining the break diameter .
We determine a best-fit break-diameter of the Kuiper belt size-distribution of 30 < D _ { b } < 90 km via a maximum-likelihood fit of our model to the observed luminosity function .
We also confirm that the observed luminosity function for m ( R ) \sim 21 - 28 is consistent with a broken power-law size distribution , and exhibits a break at m ( R ) = 26.0 ^ { +0.7 } _ { -1.8 } .