This paper investigates the performance of wireless mesh networks advocating for Opportunistic Routing (OpR) over traditional Single Path Routing (SPR). In OpR, all nodes overhearing the transmission can form a cooperation group and forward the fragments of the received message. The bigger the cooperation group, the more is the message subdivided and the harder it gets to manage this process. If poorly managed, multiple duplicate fragments can be forwarded that downgrades the overall performance and destabilizes the routing protocol. In this paper, we propose several methods for shrinking the cooperation group and define the best one that can be later used as a part of the routing protocol. Using a testbed of 15 node WiFi network deployed in the office building, we show that the cooperation group size can be significantly reduced using our proposed methods. The throughput gain of OpR over SPR reaches as high as 51% in our testbed. We also show that only 3% of the average gain is lost when shrinking the cooperation group to less than one-third of its original size. The conducted measurement and the analysis of the results form the benchmark for potential performance evaluation of the mesh networks.