This applies to version 1.5 or newer.
The EditorialConfig defines the default page thumbnail option when not explicitly specified in the directory style:
...
<EditorialConfig
xmlns="http://www.teradp.com/schemas/GN4/1/EditorialConfig.xsd"
CmdDefMis="true"
LibThumbSize="400"
MasterThumbSize="400"
DPIMinLimit="36"
DPIMaxLimit="1200"
DefaultEngraverInfo="9538m,9538m,9538m,9538m,0,1"
WordSeparator=" !\"'(),-.:;<>?[]_`"
SpellThickSrc="1"
SpellThickJust="2"
SpellColor="Red"
DefaultPagePreviewOptions="Preview480px"
>
...
If no default is provided in the EditorialConfig, the hard coded default is used (GIF format, 480px height).
See also
Page thumbnail options file structure