[INTERNAL] add getSize() in IFile interface

Implement the new Method in Eclipse and IntelliJ implementation and
using it in CreateArchiveTask

Change-Id: I1d86d64aefbbce13f845cc005fa7786bbaf9628b
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1801
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>