gistfile1.js

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

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

MySymbol.prototype.width = function() {
    return this.mySymbolItem ? Math.round(this.mySymbolItem.width) : 0;
};

MySymbol.prototype.height = function() {
    return this.mySymbolItem ? Math.round(this.mySymbolItem.height) : 0;
};

MySymbol.prototype.widthAndHeight = function() {
    var widthAndHeight = [];
    var myWidth = this.width;
    var myHeight = this.height;
    widthAndHeight.push(myWidth);
    widthAndHeight.push(myHeight);
    return widthAndHeight;
}

var firstSymbol = new MySymbol(0);
print(firstSymbol.widthAndHeight);