FINDSTR da SS64 <- Escapes and Length limits diz:
Line Length limits
Files specified as a command line argument or via the
/F:FILE
option have no known line length limit. Searches were successfully run against a 128MB file that did not contain a single <LF>.Piped data and Redirected input is limited to 8191 bytes per line. This limit is a "feature" of FINDSTR. It is not inherent to pipes or redirection. FINDSTR using redirected stdin or piped input will never match any line that is >=8k bytes. Lines >= 8k generate an error message to stderr, but ERRORLEVEL is still 0 if the search string is found in at least one line of at least one file.
Credits:
Dave Benham - List of undocumented features and limitations of FINDSTR from StackOverflow