Use the server's global max_board_size on the client

client-data/js/board.js

 function resizeCanvas(m) {
 	//Enlarge the canvas whenever something is drawn near its border
 	var x = m.x | 0, y = m.y | 0
-	var MAX_BOARD_SIZE = 65536; // Maximum value for any x or y on the board
+	var MAX_BOARD_SIZE = Tools.server_config.MAX_BOARD_SIZE || 65536; // Maximum value for any x or y on the board
 	if (x > Tools.svg.width.baseVal.value - 2000) {
 		Tools.svg.width.baseVal.value = Math.min(x + 2000, MAX_BOARD_SIZE);
 	}