The default send/receive message length of gRPC is defined here:

/** Default send/receive message size limits in bytes. -1 for unlimited. */
/** TODO(roth) Make this match the default receive limit after next release */
#define GRPC_DEFAULT_MAX_RECV_MESSAGE_LENGTH (4 * 1024 * 1024)

We can know that send message length is no limited (-1), while Server/Client can only receive 4 Mi bytes by default.

You can change receive message length to unlimited in Client:

grpc::ChannelArguments ch_args;
std::shared_ptr<grpc::Channel> ch = 
        grpc::CreateCustomChannel("localhost:50051", grpc::InsecureChannelCredentials(), ch_args);

But this doesn’t work in Server:

ServerBuilder builder;

Because in, the parameter only takes effect when it is positive:

std::unique_ptr<Server> ServerBuilder::BuildAndStart() {
    if (max_receive_message_size_ >= 0) {
      args.SetInt(GRPC_ARG_MAX_RECEIVE_MESSAGE_LENGTH, max_receive_message_size_);

So we can use builder.SetMaxReceiveMessageSize(INT_MAX); as a work-around.

BTW, check message length limit is in get_message_size_limits function.