In this paper, a cooperative Multiple-InputMultiple-Output (MIMO) scheme for wireless sensor networks is presented. We consider a network which consists of inexpensive sensor nodes transmitting data towards sinks. We take into account the hardware limitations this kind of devices typically present. Having in mind potentially high number of nodes in the network, we study the asymptotic regime, inspired by recent developments in the massive MIMO field, which simplifies the performance analysis and is shown to be sound even for relatively low number of nodes. The aim is to provide the optimal number of sensor nodes to be deployed and the duration of the channel estimation phase, having in mind energy efficiency as performance metric.