I created a quick jsFiddle which displays rather well that your code should work just fine. However you are probably using "this" incorrectly :)
https://jsfiddle.net/qoxub22u/
Html mockup
<div class="container">
<h5>First one has height of 40px</h5>
<ul style="height:40px">
<li>Test</li>
<li>Test1</li>
</ul>
<h5>Second one has height of 70px</h5>
<ul style="height:70px">
<li>Test</li>
<li>Test1</li>
<li>Test2</li>
<li>Test3</li>
</ul>
</div>
<button id="submit">Get height</button>
Jquery part
$( "#submit" ).click(function() {
/* Replace .container with ur this selector */
var height = $('.container').find("ul").filter(":first").height();
alert(height);
});