gistfile1.js

function MySymbol(input){
    this.doc = app.activeDocument;
    this.mySymbolIndex = input || 0;
    this.mySymbolItem = doc.symbolItems[this.mySymbolIndex];
}

MySymbol.prototype.hasSymbolIndex = function() {
    return this.mySymbolIndex <= this.doc.symbolItems.length - 1;
};

MySymbol.prototype.width = function() {
    if (!this.hasSymbolIndex()) { return 0; }
    return Math.round(this.mySymbolItem.width);
};

MySymbol.prototype.height = function() {
    if (!this.hasSymbolIndex()) { return 0; }
    return Math.round(this.mySymbolItem.height);
};

var firstSymbol = new MySymbol(1);

firstSymbol.width();

firstSymbol.height();