If you want to change the default thumbnail size for all folders, you can do that via Preferences.
If you only want 64x64 thumbnails in one particular folder, that isn't something you can do by default, but it can be done using a script:
If you want to change the default thumbnail size for all folders, you can do that via Preferences.
If you only want 64x64 thumbnails in one particular folder, that isn't something you can do by default, but it can be done using a script: