Maximum data you can store per key in datastores is 4 megabytes or roughly 4.000.000 bytes which is how long can a string be since 1 character takes one byte(Excluding some UTF-8 characters that uses more than one byte to store a character.).
256 kilobytes or 256.000 bytes was the old data size limit until it got increased to 4 megabytes after this announcement.
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