What is the default maximum HTTP header size (e.g. 16K)?
Where is this configured - e.g. watt.server.http.something?

For acting as server, I tested IS with 300K of header, it’s just working fine.
Not sure if any limits.
If you have a case to use very large header (>16K), you may want to review if header the right place for such large info.
or open a ticket with SAG to be sure if there is a limit/settings on the header size.

By default, there is no limit to the size of the HTTP header. You can set a limit with watt.security.max.headerLength. I believe this property was introduced in a fix to 10.1.

I have never encountered any http header limits with Integration Server. And it make sense since HTTP headers are used in many ways that may approach the size of header way higher than usual. Eg. Kerberos tickets in HTTP headers can be usually as big as 8 kb Itself. On top of it you may include singnature or any other header details.

For conformaiton, please check with Software AG support.