服务端开启server的时候设置

maxSize := 20 * 1024 * 1024
s := grpc.NewServer(grpc.MaxRecvMsgSize(maxSize), grpc.MaxSendMsgSize(maxSize))
PbDownMaxSize.RegisterPbDownMaxSizeServer(s, &server{})
s.Serve(listener)

客户端连接的时候设置

maxSize := 20 * 1024 * 1024
diaOpt := grpc.WithDefaultCallOptions(grpc.MaxCallRecvMsgSize(maxSize), grpc.MaxCallSendMsgSize(maxSize))
conn, err := grpc.Dial(endpoint, grpc.WithInsecure(), diaOpt)