There’s a similar question for OpenOffice here, but there is no answer.
It is at the same place as for all other kind of documents too. Go to menu Tools > Options > Load/Save > General. From the field Document type choose Drawing. And from the field Always save as choose the desired format.
If you really mean “Export”, I know no way to preselect a file type.
I also would like to know. He/she meant Export, which means to produce an image file of the selected draw or page. Until version 6.3, the default export type was WMF (go figure why!), and in 6.4 it is TIFF. I would like to change it to PNG, but I see no (obvious) way. Probably the option is hidden in a mysterious Easter egg.
Jorge