gistfile1.js

function mySymbol(input){
    var idoc = app.activeDocument;
    if (input <= idoc.symbolItems.length-1) {
        alert(idoc.symbolItems.length);
        if (input) {
            this.mySymbolIndex = input;
        } else { this.mySymbolIndex = 0; }; //set up default symbol to first one if none is defined
        this.width = function () {
            var symbolWidth = Math.round(idoc.symbolItems[this.mySymbolIndex].width);   //convert float to int
            return symbolWidth;
        }  
        this.height = function () {
            var symbolHeight = Math.round(idoc.symbolItems[this.mySymbolIndex].height); //convert float to int
            return symbolHeight;
        }
    } 
    else { 
        alert('There are only '+idoc.symbolItems.length+'symbols in your library, you selected '+input+'try a lower number');
        this.height = 0;
        this.width = 0;
    }
}

var firstSymbol = new mySymbol(1);
firstSymbol.width();