Dynamically Keep Dropdownlist Same Size As Input Box - html

Question

I am working on an ASP.NET, Bootstrap 3 website and have a problem with changing the size of the dropdownlist.
I can change it manually in the CSS to match the size of the other input boxes but I don't want to edit it manually every time I need to change it.
Is there a way to always keep the width the same between the dropdownlist and the other input boxes?
Here is what it is doing:
iPhone 6 vs
iPhone 5: Problem. The problem is that the dropdownlist doesn't get resized within the div.
And here is an example of my code:
<section class="features">
#using (Html.BeginForm("Create", "UsersAdmin", FormMethod.Post, new { #class = "form-horizontal", role = "form" }))
{
#Html.AntiForgeryToken()
<h4>Create a new account</h4>
<div class="row">
<div class="col-lg-4 col-md-4 col-sm-12 col-xs-12">
<!--ALL THESE INPUT BOXES ADJUST AUTOMATICALLY-->
<div class="form-group">
#Html.LabelFor(m => m.Address, new { #class = "col-md-5 control-label" })
<div class="col-md-7">
#Html.TextBoxFor(m => m.Address, new { #class = "form-control" })
#Html.ValidationMessageFor(m => m.Address, "", new { #class = "text-danger" })
</div>
</div>
<div class="form-group">
#Html.LabelFor(m => m.City, new { #class = "col-md-5 control-label" })
<div class="col-md-7">
#Html.TextBoxFor(m => m.City, new { #class = "form-control" })
#Html.ValidationMessageFor(m => m.City, "", new { #class = "text-danger" })
</div>
</div>
<!--HERE IS THE DROPDOWLIST THAT NEEDS TO BE ADJUSTED DYNAMICALLY-->
<div class="form-group">
#Html.LabelFor(m => m.State, new { #class = "col-md-5 control-label" })
<div class="col-md-7 dropdownlist">
#Html.DropDownListFor(m => m.State, Steel_Tech.STATE.States, "--Select a State--", new { #class = "dropdownlist" })
#Html.ValidationMessageFor(m => m.State, "", new { #class = "text-danger" })
</div>
</div>
<div class="form-group">
#Html.LabelFor(m => m.Zip, new { #class = "col-md-5 control-label" })
<div class="col-md-7">
#Html.TextBoxFor(m => m.Zip, new { #class = "form-control" })
#Html.ValidationMessageFor(m => m.Zip, "", new { #class = "text-danger" })
</div>
</div>
</div>
</div>
<div class="form-group">
<input type="submit" class="btn btn-default" value="Create" />
</div>
}
#section Scripts {
#Scripts.Render("~/bundles/jqueryval")
}