With the advent of the new X-ray missions Chandra and XMM-Newton, high-resolution spectroscopy has become available for studies of stellar coronae. Individual lines can be used as diagnostic tools for measuring densities and temperatures in coronal plasma. In addition to the overall X-ray luminosity, spectroscopic properties can be used for a classification of coronal X-ray emitters. In this paper we focus on density diagnostics measured with the He-like triplets and measurements from ratios of H-like and He-like lines yielding a characteristic plasma temperature. We test the assumption of optically thin plasma emission using the Fe xvil emission lines at 15.03 and 15.27 Å. We find that tha assumption of optically thin plasma is valid for all targets in our sample. Deviations from theoretical ratios can be explained by unidentified blending of the 15.27 Å line. From the results of our analysis we conclude that the stars with low activity are all quite similar with low temperatures and low densities. The plasma with a low overall emission measure is not connected with high density regions but occupies larger volumes. For the active stars we measure higher temperatures but two scenarios for the densities. We measure relatively low densities for all active RS CVn systems and higher densities for all the other active stars except for β Ceti. We conclude that large available volumes (as in RS CVn stars and β Ceti) are completely filled with emitting plasma and only limited volumes in combination with high emission measures require mechanisms producing plasma with higher densities.