getSize
Gets the file's size (in bytes). The size is retrieved through the value of the %G_FILE_ATTRIBUTE_STANDARD_SIZE attribute and is converted from #guint64 to #goffset before returning the result.
It is an error to call this if the #GFileInfo does not contain %G_FILE_ATTRIBUTE_STANDARD_SIZE.
Return
a #goffset containing the file's size (in bytes).