I'm not too sure about this one. Personally, it seems to me like when people are talking or writing about natural disaster, like floods, earτhquakes, etc, both in English and in Greek, big covers it way better than large does, because it does have the meaning of major, Important as well. Not just big in size, but of importance as well. That's just my opinion though ^.^
As a native English speaker, to me, "the large flood" does not necessarily sound incorrect, but "the big flood" definitely does sound better. As a side note, I recommend adding "the great flood" as an acceptable option (As Dimitra hints with her comment, "great" is another adjective that conveys both an idea of "big" and also "major," plus for English speakers, "the great flood" sounds familiar due to its Biblical reference).
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I would think "large flood" works as well. There are times when you are describing a flood that is not necessarily a natural disaster such as a flood in the house from a pipe bursting. In a case like this saying "large" would be correct ( in my opinion). " there was a large flood in my kitchen from a broken sink". works just as well as "big" and is not necessarily "great" in this instance. Since the phrase is "η μεγαλη πλημμυρα" we don't have full context of what is being spoken about. Hence, I believe "large, big, great" are all acceptable.
"Large" is really uncommon in this context and as we see in the comments above, "large flood" is not a safe bet, really. As far as "important" is concerned, it's mostly used in cases like "the big day, the big match" etc, but, in any case, the Greek word for "important" is "σημαντικός", which fits better in other sentences and has been used as the main translation in those cases.
Maybe it's more academic sounding, but it's correct English. It should not be marked wrong.
The World's Largest Floods, Past and Present - USGS Publications Warehouse PDF
https://pubs.usgs.gov › pdf › circ1254