If the values you're going to store to that column are sometimes 10 characters and sometimes 50 characters long, it would seem to me that they're actually two different kinds of data, and you should perhaps change your database model to store them in two different columns.
If, on the other hand, the length of all values varies between 10 and 50 characters, then a VARCHAR(50) would work quite nicely for that. If the 50 character limit could be (significantly) increased in the future, then perhaps you should use TEXT instead. But it's you who knows the data, so you need to decide that.
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