var docRef = app.activeDocument;
var spacing = 20;
var greatestWidth = 0;
var startX = 0;
var startY = 792;

var xPos = startX;
var yPos = startY;

for (var i = 0; i < docRef.symbols.length; i++) {
	var mySymbol = docRef.symbols[i];
	var myInstance = docRef.symbolItems.add(mySymbol);

	myInstance.top = yPos;
	myInstance.left = xPos;
	
	yPos -= myInstance.height + spacing;
	
	if (myInstance.width > greatestWidth) {
		greatestWidth = myInstance.width;
	}
	
	if (yPos <= 0) {
		xPos += greatestWidth + spacing;
		yPos = startY;
	}
}