The DM_LISTGETDATASIZE message is sent to the dialog manager to retrieve the size of the data associated with an item in a
DI_COMBOBOX or DI_LISTBOX list.
Param1
The ID of the dialog item for which the item data is retrieved.
Param2
Index of the item for which the data is retrieved.
Return
Size of the data value the plugin has associated with the item using DM_LISTSETDATA messages.