Add the following CSS to your html page, it will apply an overflow setting and set the width of the auto complete list:
/**causes the autocomplete to align left and overflow with vertical scroll bar **/
ul
{
text-align:left;
width: 150px;
}
.ui-autocomplete
{
max-height: 200px;
overflow-y: auto;
overflow-x: hidden;
text-overflow: ellipsis;
white-space: nowrap;
}