$($('.ui-autocomplete-input')[1]).find('.ui-menu-item').find('.ui-corner-all').css('font-size','12px');

$("#input1").autocomplete({
    delay: 0,
    minLength: 0,
    source: ["One", "Two", "Three"]
}).data("autocomplete")._renderItem = function(ul, item) {
    return $("<li class="li-class"></li>")//added list class here
       .data("item.autocomplete", item)
       .append("<a class="a-class">" + item.label + "</a>") //added the anchor class here.
       .appendTo(ul);
};​