Water mist generated during a rocket launch is thought to protect the rocket and payloads from acoustic noise. The size of mist particles is essential to understanding the effect on noise reduction. A two-wavelength multi-static lidar was developed for measuring water mist size at the launch site. The lidar determines particle size from signals at three scattering angles at two wavelengths. The method was tested with artificial mist and applied to the Japan Aerospace Exploration Agency's H-IIA/B large-scale rocket launches. The measured particle size near the outside edge of the mist cloud was 3.5-5 μm in diameter. The extinction coefficient at 532 nm derived using the Klett backward inversion method was 100-200 km-1. The estimated liquid water content (LWC) was ∼0.3 g/m3. The extinction coefficient was high, but the LWC was comparable to that of the water clouds.