Thumbnails #N (Width x Height): displays thumbnails of maximum Width x Height pixels. The default option is 125x125.
Thumbnails (Custom): displays thumbnails with the maximum Width and Height defined by user. Clicking the Thumbnails (Custom) menu item displays the selector of the desired thumbnails size to display:
The Thumbnails size value may lay in the range from 50 to 2500 pixels.
You can also use CTRL+Mouse Wheel or a Touchpad to change the size of the thumbnails in the Thumbnails view:
The information of this topic may also be applied to the Search Results View.