Miguel - I am assuming that you mean the size of an edoc file attached to a document?  If so, here is a Workflow SDK code snippet that will do that.  The token exposed by the script activity is called 'FileSize' and represents the size in bytes of the edoc file.  This is a 9.2 SDK Script activity using RA.  If you have an earlier version of Workflow and need the LFSO code then let me know...

        Protected Overrides Sub Execute()
            'Write your code here. The BoundEntryInfo property will access the entry, RASession will get the Repository Access session

            If Me.BoundEntryInfo.EntryType = EntryType.Document Then
                Dim docInfo As DocumentInfo = DirectCast(Me.BoundEntryInfo, DocumentInfo)

                Me.SetTokenValue("FileSize", docInfo.ElecDocumentSize)

                docInfo.Dispose

            End If

        End Sub

NOTE: You can also use the 'Find Entry' workflow activity and set the 'Additional Properties' to expose the edoc filesize.  Perhaps that would be easier and more efficient than the SDK script?