Photoshop pros differ in their choice of thumbnail size on the Layers, Paths, and Channels palettes. Some usually run with small thumbnails, some with none, others with medium or large. Once in a while, you need to alter that size. Maybe an image with lots of layers would be easier to work on if the thumbnails were hidden, allowing more layers to fit within the same palette space. On the other hand, it’s often helpful to get a better look at thumbnails on the Channels palette while focusing on one color.
While you can always select Palette Options from the Layers, Paths, or Channels palettes flyout menu, then set the thumbnail size there, there’s an easier method: Right-click on the empty area of the palette beneath the last layer, channel, or path (single-button Mac users CTRL-click). Up will pop choice of thumbnail size.