We present high-speed photometry of the eclipsing cataclysmic variable CSS131106 J052412+004148. We determine the system parameters by modelling the eclipse light curve using the photometric eclipse method, in which the mass ratio is determined from the relative timings of the white dwarf and bright spot eclipses. Despite the blended white dwarf and bright spot ingress, typical of longer period cataclysmic variables, we perform simulations that show we are able to reliably constrain the component masses. We find a mass ratio of $q = 0.81 \pm 0.06$ and inclination $i = 78.5 \pm 0.7$ degrees. The white dwarf and donor masses were found to be $M_{w} = 0.72 \pm 0.04$ ${\rm M}_{\odot }$ and $M_{d} = 0.58 \pm 0.06$ ${\rm M}_{\odot }$ respectively. The white dwarf temperature was estimated to be $T_{\rm eff} = 18~500 \pm 2000$ K, implying a moderate accretion rate of $\dot{M} = 3 \pm 1 \times 10^{-10}$ ${\rm M}_{\odot }$ yr$^{-1}$. The donor in CSS131106 J052412+004148 joins two other long-period cataclysmic variables (IP Peg and HS 0220+0031) in being unusually small for its mass, even when compared to detached M-dwarfs. The donors in all three systems are also unusually cool for their mass. We discuss possible explanations for the small radii and cool temperatures of the donors in these systems, but find no viable explanation for their properties.