JavaScript对象限制大小？

Question

当然我做错了什么，但我无法实现什么？ 我的问题是我正在做一个AJAX请求，我得到了响应，但是当我要把它放在一个JavaScript对象中它失败时，如果它的大小有点大。无论我是尝试JSON还是XML。这是我的代码，我非常感谢任何帮助：

function DataFlow (Target, Txt, Tid, Rid){  
    var httpObj, obj, row, htm, col, par;
    this.tid = Tid;
    this.rid = Rid;

    if (window.XMLHttpRequest)
        httpObj = new XMLHttpRequest();
    else if (window.ActiveXObject)
        httpObj = new ActiveXObject("Microsoft.XMLHTTP");
    else
        window.alert ("Navegador nao suporta AJAX");        

    if (httpObj) {
        httpObj.onreadystatechange = function (){
            var txt, nrow;
            if (this.readyState == 4){
                if (this.status != 200)
                    window.alert ("Pagina nao encontrada: " + Target);
                else {  
                    txt = httpObj.responseText;
                    if (txt == null){
                        window.alert ("Sem Resposta");
                        return;
                    }

                    window.alert ("Size: " + txt.length + " bytes");
                    //No Problems Here
                    obj = eval ('(' + txt + ')');
                            //Sintaxe Error for Eval or XML for sizes over 300Kb
                    if (obj == null)
                        window.alert ("JSON!");

            }
        };
        httpObj.open("POST", Target, true);
        httpObj.send(Txt);
    }
}