Measuring MY dipstick on my M25 is NOT going to do YOu any good, since I have a different engine. I suggest, since it appears your dipstick is broken, that you simply buy a new one. Either find a local Universal dealer, or go to Marine Diesel Engines and Engine Parts from Torresen Marine's Marine Diesel Direct - Universal - Volvo - Westerbeke - Yanmar - MASE
(a handy bookmark if ever there was one!).
Stu Jackson, C34, 1986, M25 engine, Rocna 10 (22#)