We present an analytical model that quantitatively describes the physics behind shunting in thin-film photovoltaics and predicts size-dependent effects in the I/V characteristics of solar cells. The model consists of an array of micro-diodes and shunt in parallel between the two electrodes, one of which mimics the TCO and has a finite resistance. We introduce the concept of the screening length L, over which the shunt affects the electrical potential of the system. The nature of this screening is that the system generates currents in response to the point perturbation caused by the shunt. L is expressed explicitly in terms of the system parameters. We find the spatial distribution of the electrical potential in the system and its I/V characteristics. The measured I/V characteristics depend on the relationship between the cell size l and L, being markedly different for the cases of small (l<<L) and large (l>>L) cells. This model is verified experimentally; good agreement is obtained.