Edit in JSFiddle

<input type="text" id="Username" />

$(document).ready(function () {

        var userArray = ["Fabregas",
        "Messi",
        "Ronaldinho",
        "Henry",
        "Cazorla",
        "Goetze",
        "Reus",
        "Luiz",
        "Rooney",
        "Ronaldo",
        "Iniesta",
        "Silva",
        "Gerrard",
        "Scholes"];

        $("#Username").autocomplete({
            source: userArray
        });
    });

.ui-autocomplete {
		max-height: 100px;
		overflow-y: auto;
		/* prevent horizontal scrollbar */
		overflow-x: hidden;
	}
	* html .ui-autocomplete {
		height: 100px;
	}