What is the format of the TLS extension field?
Section 7.4.1.2 of RFC 5246 defines the following ClientHello:


  struct {
      ProtocolVersion client_version;
      Random random;
      SessionID session_id;
      CipherSuite cipher_suites&lt;2..2^16-2&gt;;
      CompressionMethod compression_methods&lt;1..2^8-1&gt;;
      select (extensions_present) {
          case false:
              struct {};
          case true:
              Extension extensions&lt;0..2^16-1&gt;;
      };
  } ClientHello;



As you can see, you can add up to $2^{16}=65536$ bytes of extensions to your client hello in a list format as per section 4.3.