How to resize input/submit box in Bootstrap3 - css

Question

I have a fairly simple large input along with a submit box using the bootstrap3 framework, which is displaying perfectly on desktop/tablet resolutions, but viewed under a mobile viewport the submit button slides under the input box, and it looks rather ugly.
How do I either shrink the input/submit buttons to resize appropriately so they don't tile on top of each other, or alternatively if that's not possible have the submit button center align on mobile screensize only?
<div class="col-md-8 col-md-offset-4">
<div class="input-group input-group-lg">
<form class="navbar-form navbar-left" role="search" action="../includes/some.php" method="POST" id="navbar">
<div class="form-group">
<input type="text" class="form-control" id="focusedInput" autocomplete="off" maxlength="17" placeholder="Enter number" name="number">
</div>
<button type="submit" class="btn btn-primary submit-btn" data-toggle="tooltip" data-placement="bottom" title="something">SUBMIT</button>
</form>
</div>
</div>
Fiddle is here - http://jsfiddle.net/uradA/3/

Answer 1

Do something like this as taken from Button addons:
<div class="col-lg-6">
<div class="input-group">
<input type="text" class="form-control big">
<span class="input-group-btn">
<button class="btn btn-primary big" type="button">Submit</button>
</span>
</div>
</div>
Where big is:
.big {
font-size: 30px;
height: 80px;
}
Here is the updated example: jsfiddle.net/uradA

Answer 2

try this
<div class="col-md-8 col-md-offset-4">
<div class="input-group input-group-lg">
<form class="navbar-form navbar-left" role="search" action="../includes/some.php" method="POST" id="navbar">
<div class="col-xs-12 col-sm-8 form-group">
<input type="text" class="form-control" id="focusedInput" style="width:100%" autocomplete="off" maxlength="17" placeholder="Enter number" name="number"/>
</div>
<div class="col-xs-12 col-sm-4 form-group">
<button type="submit" class="btn btn-primary btn-block submit-btn" data-toggle="tooltip" data-placement="bottom" title="something">SUBMIT</button>
</div>
</form>
</div>
</div>