How do I get more than 64 characters of message from serial
First: There is no such concept of a message on the Serial bus, unless you implement it yourself. On hardware level only individual bytes are transmitted, and you don't even know, when each byte will be transmitted exactly. Since there are only individual bytes in the first place, there is no limitation on how many bytes can be transmitted.

Yes, the Serial library has a limited buffer of 64 bytes, but as Majenko wrote it is not intended for buffering all bytes for one of your messages. It should only buffer the bytes, until the main sketch can read them from the buffer. On reading the bytes will be removed from the buffer, leasing buffer space for new characters.

The buffer size of the Serial library can be increased by changing the numbers in HardwareSerial.h in the following lines

#define SERIAL_TX_BUFFER_SIZE 64

#define SERIAL_RX_BUFFER_SIZE 64


But this is not really the solution to your problem. When first filling the buffer and then read the whole message you just arbitrarly 
constrain yourself to a insufficient solution.

Instead of waiting for all the messages bytes (and finally filling up the buffer) you should steadily check, if new bytes are available and then load them into your own buffer. You can use a delimiter byte to mark the end of a message, mostly "\n" (newline character) is used for this. Copy the bytes into your own buffer, until you read the delimiter byte. Then use the message as a whole for your application. Don't forget to clear your buffer afterwards.

The examples for the Serial library, especially the SerialEvent example show how to do this in your code.