Flow size estimation is highly important and beneficial to various applications, including traffic engineering and anomaly detection. Considering the resource constraints and performance requirements in place, sketches are widely used to accomplish this task. However, when sketches are applied in real-life networks, the measurement performance is often decreased due to many practical reasons, including partial deployment of sketches, unique traffic characteristics, etc. In this paper, we present sRouting, a practical framework that aims to better utilize the deployed sketches. sRouting improves the measurement performance by optimizing which flows are monitored and where. We first investigate the relationship between the accuracy of a given sketch and the total number of packets, then formulate the offline problem in sRouting as an integer linear programming problem. To solve this problem efficiently, we devise a rounding-based algorithm and provide its performance guarantees. Furthermore, to handle dynamic changes in the network, we design an online adjustment algorithm capable of responding appropriately to these changes. Through experiments using real traces and typologies, we demonstrate that sRouting can significantly improve the volume of monitored traffic and reduce the measurement error without negatively impacting the network throughput.