.selectbox
{
    border-right: #d4d0c8 1px solid;
    border-top: #d4d0c8 1px solid;
    border-left: #d4d0c8 1px solid;
    border-bottom: #d4d0c8 1px solid;
    font-size: 8pt;
    font-family: tahoma;
    color: black;
    background-color: white;
    width: 150px;
    height: 303px;
}

<select class="selectbox" id="folders" name="folders" size="10">
...
</select>

overflow: auto;