[API Gateway] What is the maximum size of the connection ID used in @connections commands?

I plan to store the connection IDs used in @connections commands in DynamoDB. cf., https://docs.aws.amazon.com/apigateway/latest/developerguide/apigateway-how-to-call-websocket-api-connections.html

I would like to know the maximum size of a connection ID for calculating the Capacity Unit of DynamoDB. Is there a specification for the maximum size of Connection IDs?