Also known as galactic clusters, open star clusters come in many shapes and sizes. A rich cluster may comprise several thousand stars, while a sparse one may contain only a dozen or so. Some clusters, such as the Pleiades (M45) in Taurus, are large and bright enough to see with the naked eye. Most, however, require a telescope.
Large open clusters, such as the Beehive (M44) in Cancer, are best viewed using the low magnification of a wide-angle eyepiece. Small, compact clusters, such as the Jewel Box (NGC 4755) in Crux, can stand much higher magnifications. By increasing the magnification, you gain contrast, but you should avoid increasing it so much that the cluster becomes large and sparse. Aim for a balance between image scale, contrast, and detail.
A star cluster's overall visual presence is determined by its concentration—how compact or loose it is—as well as the distribution and individual magnitudes of its stars. Compact clusters made up of faint stars near the resolution limit of your telescope will appear nebulous, while large rich clusters can be hard to distinguish from background stars.