In Vincent Maraia's The Build Master (recommended) there's a helpful little chart on the size in lines of code for Windows NT:
Ship Date Product Dev Team Size Test Team Size Lines of code (LoC) Jul-93 NT 1.0 (released as 3.1) 200 140 4-5 million Sep-94 NT 2.0 (released as 3.5) 300 230 7-8 million May-95 NT 3.0 (released as 3.51) 450 325 9-10 million Jul-96 NT 4.0 (released as 4.0) 800 700 11-12 million Dec-99 NT 5.0 (Windows 2000) 1,400 1,700 29+ million Oct-01 NT 5.1 (Windows XP) 1,800 2,200 40 million Apr-03 NT 5.2 (Windows Server 2003) 2,000 2,400 50 million