[e1f]

function chsz(xobj, id, w, h, speed){
    var t1, t2;
    var obj=document.getElementById(id);
    var n=x=Number(obj.offsetWidth), m=y=Number(obj.offsetHeight);
 
    xobj.onmouseover=function(){
        clearInterval(t2);
        t1 = setInterval(function(){
            if(n<w){
                n+=5;
                obj.style.width=n+'px'
            }
            if(m<h){
                m+=5;
                obj.style.height=m+'px'
            }
            if(n>=w&&m>=h) clearInterval(t1)
        }, speed);
    };
    xobj.onmouseover();
    xobj.onmouseout=function (){
        clearInterval(t1);
        t2 = setInterval(function(){
            if(n>x){
                n-=5;
                obj.style.width=n+'px'
            }
            if(m>y){
                m-=5;
                obj.style.height=m+'px'
            }
            if(n<=x&&m<=y) clearInterval(t2)
        }, speed);
    };
    return
}

А так сработает?
P.S. .onMouseover и .onmouseover -- разные методы