How can I increase the size of the receiving sms Buffer in GSM

Recently I am working on programming IoT Project, to receive messages from the phone. That part worked, but the problem that I am currently facing is that the message does not arrive fully because of the size of the buffer. That’s why am asking a way to increase the buffer size. I have tried to change it manually, but the problem was not solved.

The Code is like

    char smsBuffer[250];
    // Retrieve SMS value.
    uint16_t smslen;
    // Pass in buffer and max Glen!
   if (sim800l.readSMS(slot, smsBuffer, 250, &smslen)) 
   {
    smsString = String(smsBuffer);
    Serial.println(smsString);
   }