How to increase HTTP Request Buffer Size

https://startingelectronics.org/tutorials/arduino/ethernet-shield-web-server-tutorial/SD-card-IO/
This is a great example to learn how to host a web site and exchange data via ajax and HTTP Request. I am a novice user and have used this example as a baseline to implement an editable table. My last hurdle is the buffer length. This example specifies buffer length as 60 bytes (#define REQ_BUF_SZ 60). I would like to bump this up to about 1200 bytes so I can transmit all my table data cells to the arduino. I have tried increasing the buffer size and have been unsuccessful - my software permanently stalls. From my research the buffer size should be limited to 2048 bytes. Can anyone please help me out here. After many hours of interNet research I am totally out of ideas.
Thanks. Any help will be appreciated

#define REQ_BUF_SZ 60 // size of buffer used to capture HTTP requests

char HTTP_req[REQ_BUF_SZ] = {0}; // buffered HTTP request stored as null terminated string

What Arduino are you using ? If it is an Uno you only have 2K of RAM for the variables including buffers in your code. The SD card will require a buffer of 512 bytes and the Ethernet card will also require a buffer so that is likely to leave you struggling to have a buffer of 1200bytes. Note that the SD card buffer is allocated dynamically so won't show up in the compiler information when you build the project, but if there is not enough RAM available when youtry to open the file it will simply fail.
There should be no need to send all the data in one go, an ethernet packet is limited to 1518 bytes, this includes the overheads so the amount of data you can normally put into a single packet is less than 1500 bytes.

This old thread may help to understand https://forum.arduino.cc/index.php?topic=134868.15

#include <EthernetUdp.h>
SendMessage(String Message){         
  char charBuf[20];
  Message.toCharArray(charBuf, 20);  
  Udp.beginPacket(ipR, 11000);
  Udp.write(charBuf);
  Udp.endPacket(); 
}

passing an array of 11000 bytes

  Udp.beginPacket(ipR, 11000);

That does not pass an array of 11000 bytes, the arguments to beingpacket ar ipaddresss and port number so it is using port number 11000.
It is passsing charbuff which has a size of 20 bytes.

I’m on a Mega

Go to https://www.xxxx.com/system/console/configMgr. Search CQSE and click on the link to the Day CQSE HTTP Service. A new pop-up window will be opened. Change the value of "Request Buffer Size" to the size you need (the default is 8KB)...

www.xxxx.com ?