Overhead transmission line sag is a crucial parameter that needs to be measured for safe and efficient power transmission. Due to this reason, real time measurement of transmission line sag is necessary in a smart power grid. The ability of smart grid to facilitate real-time monitoring of different power system parameters allows sag measurement and monitoring of overhead transmission lines to be integrated with smart grid system. In this paper, two sag measurement methods based on millimeter wave (mmWave) signals are proposed. The performance of the proposed methods is analyzed and compared for practical 132 kV, 230 kV and 400 kV overhead power transmission lines. The first method uses single transceiver, whereas, the second method uses multiple transceivers for measuring sag. Simulation results demonstrate that the second method shows significantly better accuracy than the first method. The performance of the communication network for establishing sag related information exchange among the devices in the proposed methods is also evaluated in this paper. Moreover, trade-offs between latency and sensitivity with bandwidth, and latency and percentage average error with number of samples are also rigorously investigated.