If we had a word for “inland freshwater sea” then we would be able to distinguish between a lake and a _lake_. The best we can do right now is add “great” preceding it!
In the end, the disctinction based on size is a wonky one and makes no sense in the long run so I don't mind having large lakes that are bigger than some seas
- die See (fem.), alt. das Meer (neutr.) - the sea (a saltwater ocean)
- der See (masc.) - the lake (an inland freshwater lake)
But then again, "das Meer" is sometimes used for freshwater lakes when connotated with some context, e.g. "das Schwäbische Meer" as an alternative name for "Lake Constance".