How do I set up Epson Scan so that it properly Thumbnails my 120/220 film images?

You must select the film size before scanning.

To view the thumbnail previews of images from 120/220 medium format film, click the Configuration button, click the Film Size tab, and then select the correct size as the Medium Format Film Size setting.

The thumbnail preview feature for 120/220 film supports the following 5 common medium format film sizes (all values are in centimeters): 6x4.5 (default), 6x6, 6x7, 6x8, and 6x9. Note that 120/220 medium format film is not supported in the Full Automatic Mode.