property size

This property contains the absolute size of the attached stream. For a read only stream this property is read only.

var fileStreamObj = shell.serviceManager.basics.fileStream;
var streamReaderObj = shell.serviceManager.basics.rawStreamReader;

fileStreamObj.openForRead("c:\\temp\\test.txt");
streamReaderObj.stream = fileStreamObj;
streamReaderObj.seekRelative(4);

var size =  streamReaderObj.size;

fileStreamObj.close();