The layer thumbnail images that are shown in Photoshop are small. Really small. If you want them to be bigger, there is a trick that allows to increase the size of the layer thumbnail images. To do this, click on the very tiny and easy-to-miss arrow from the Layers menu indicated by the red arrow in the image below.

The arrow to click on to edit layer thumbnail size

Once you click on this, select "Palette Options..." from the menu. This will open a window that allows you to select the thumbnail size. There are two options that are bigger than the default. Just select a radio button next to one of the bigger sizes and hit the "OK" button and you'll have bigger thumbnail images for your layers.

This works in CS3, I'm not sure about other versions.