With the OpenSSL3 capability to add new types of TLS (1.3) groups via providers, new cryptographic mechanisms can be added changing the contents and lengths of different TLS (1.3) fields. A concrete example is the addition of PQC algorithms with large KEM (ciphertext) structures, already going beyond the limits imposed by this constant:

As discussed here, not changing this constant means that some PQC algorithms simply cannot run within OpenSSL3 just because of this constant. The TLS specification does permit higher values.

Before merely proposing a pragmatically simple change-of-constant (to 30000 as that value served the OQS integration well so far) the computer scientist in me would prefer a discussion on a possible (probably provider-driven) general mechanism for this. I'd surely at least invite @mattcaswell @dstebila @christianpaquin @romen @t8m to chime in. The more general question may be whether more than just SERVER_HELLO_MAX_LENGTH should be updated/become updateable.