This paper considers possibilities of estimating the sizes of ice particles in cirrus clouds using measurements at two radar millimeter wavelengths. The radar frequencies considered are those at the transparency ''windows'': 35, 94, 140, and 215 GHz. It is shown that measurements of reflectivity differences at 35 and 215 GHz and at 94 and 215 GHz could potentially be used to estimate particle median sizes from about 0.2 to 0.4 mm. However, the sensitivity of reflectivity differences to particle shapes and orientations will keep the expected error of such an estimation at not less than about 0.1 mm. Measurements of differences in circular depolarization ratios at 35 and 94 GHz, 35 and 215 GHz, 94 and 215 GHz, and 35 and 140 GHz can also be used to estimate particle sizes, and the combination of 35 and 215 GHz can lower the limit of estimated median sizes to about 0.1 mm. It is also shown in the paper that an equal volume sphere approach, being a reasonable approximation for describing backscatter by small ice nonspherical particles, gives larger errors when describing backscatter by larger non-Rayleigh particles. Backscatter by these bigger particles shows significant dependence on particle orientation, which results in relatively large errors of sizing. ¿ American Geophysical Union 1993