Constant MAX_STARTUP_FRAME_SIZE 

pub const MAX_STARTUP_FRAME_SIZE: usize = 10_000;

Maximum size of a startup frame accepted directly from a client.

Matches PostgreSQL’s MAX_STARTUP_PACKET_LENGTH, so any client that can complete a PostgreSQL handshake can complete this one. A startup frame carries only the protocol version and the connection parameters, of which options is the only one a client can make large.