Możesz nadać mu szerokość:
select[name="sel"] {
width: 300px;
}
<option value="10">long description here long description here long description here long description here
</option> <option value="13">long description here long description here long description here long description here
</option>
Albo tak:
<option value="13" title="long description here long description here long description here long description here">long description here long descripti...
</option>