I took Jezzipin's answer and made it so that if you are scrolled when you refresh the page, the correct size applies. Also removed some stuff that isn't necessarily needed.
function sizer() {
if($(document).scrollTop() > 0) {
$('#header_nav').stop().animate({
height:'40px'
},600);
} else {
$('#header_nav').stop().animate({
height:'100px'
},600);
}
}
$(window).scroll(function(){
sizer();
});
sizer();