In this paper, we present a systematic study of the uplink capacity and coverage of pico-cell wireless networks. Both the one dimensional as well as the two dimensional cases are investigated. Our goal is to compute the size of pico-cells that maximizes the spatial throughput density. To achieve this goal, we consider fluid models that allow us to obtain explicit expressions for the interference and the total received power at a base station. We study the impact of various parameters on the performance: the path loss factor, the spatial reuse factor and the receiver structure (matched filter or multiuser detector). We relate the performance of the fluid models to that of the original discrete system and show that the fluid model provides a bound for the discrete one.