locate -r ".*/Thumbs.db" > ~/delme-thumbs.txt
Which queries the mlocate database for the thumbnail files. If you're not running mlocate, you would use the find command here.
cat ~/delme-thumbs.txt | tr \\n \\0 | xargs -0 du -hsc
This will show you how much each file is taking up and include a sum of the total size. Finally, you might want to free these files with the following command:
cat delme-thumbs.txt | tr \\n \\0 | xargs -0 rm