The microphysical properties of an optically thin (τ ≤ 0.12) cirrus layer formed by dispersing contrails above Boulder, Colorado, have been examined in a ground-based study. Backscatter and depolarization lidar measurements at 0.532 ¿m were used to characterize the cloud morphology, while near-infrared (0.9 to 1.7 ¿m) spectroscopy was used to measure zenith scattered sunlight from the ice particles. The spectra are used to infer the particle effective diameter from published size and shape dependent scattering calculations. The measurements indicate growth in particle size from average effective diameters less than 20 ¿m in isolated contrails, to more than 40 ¿m as the contrails coalesced to form a 1--2 km thick cirrus layer. The retrieved size distributions are consistent with published measurements of contrail-cirrus size distributions from in situ observations, and suggest that the lidar/near-infrared spectroscopy combination can provide a useful method for deriving radiatively important information regarding optically thin (τ ≤ 0.1) cirrus under conditions where the single-scattering approximation is valid.