My games rarely last more than 200 years, so the largest I can remember is about 400 alive and 1000 dead.
AFAIK most dynasties never really became particularly large with "official" members, due to the practice of cadet dynasties and so on with second and subsequent sons.
Though if real dynasties adhered to the CK2 system, some of the largest and oldest would probably number many thousands, with many perhaps not actually being extinct.
I've seen screenshots on these forums of some spectacularly huge dynasties. One person had a child born with 4k prestige thanks to dynasty prestige, they had spread their dynasty all over the map.