[Zuenf]

<ul>
    <li>
        <label>Width</label>
        <input type="text" name="width" id="widh_text_one" class="size_text" ></span>
    </li>
    <li>
        <label>Height</label>
        <input type="text" name="height" id="height_text_two" class="size_text" ></span>
    </li>
    <li>
        <label>Results</label>
        <input type="text" name="results" id="result" class="size_text_" value=""></span>
    </li>
</ul>
<script>
var heightInput = document.getElementById('height_text_two'),
    widthInput = document.getElementById('widh_text_one');
function consider(){
var height = parseInt(heightInput.value),
     width = parseInt(widthInput.value),
     pieces = 'N/A';
if (height && width) {
    if (height >= 900 && height <= 1100 && width >= 800 && width <= 1000){
        pieces = 53;
    } else if (height >= 500 && width >= 600) {
        pieces = 52;
    }
document.getElementById('result').value = pieces;
}
}
widthInput.onkeyup = heightInput.onkeyup = consider;
</script>