if searches is your div id then try following code
function zoomings(optionSel)
{
var OptionSelected = optionSel.selectedIndex;
var val = optionSel.options[OptionSelected].text;
var div = document.getElementById ("searches");
div.style.zoom = val;
}
<span>Zoom</span><select id='zooming' onchange="zoomings (this);">
<option value="25%">25%</option>
<option value="50%">50%</option>
<option value="75%">75%</option>
<option value="100%" selected="selected">100%</option>
<option value="150%">150%</option>
<option value="200%">200%</option>
<option value="250%">250%</option>
<option value="300%">300%</option>
</select>