Center fieldset using Foundation - asp.net-mvc

Question

Im trying to use Foundation grid in my application but I cannot simply center the fieldset. I try to put in the center of the screen on mobile device and desktops, but it just looks weird. Any idea why?
Please, note that Im new to Foundation and I have no experience with Bootstrap
Code below:
#using (Html.BeginForm())
{
#Html.AntiForgeryToken()
#Html.ValidationSummary(true)
<div class="row">
<div class="large-6 large-centered columns">
<fieldset>
<legend>Logging into Review Platform</legend>
<div class="editor-label">
#Html.LabelFor(model => model.UserName)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.UserName)
#Html.ValidationMessageFor(model => model.UserName)
</div>
<div class="editor-label">
#Html.LabelFor(model => model.Password)
</div>
<div class="editor-field">
#Html.EditorFor(model => model.Password)
#Html.ValidationMessageFor(model => model.Password)
</div>
<p>
<input type="submit" value="Login" class="button" />
</p>
</fieldset>
</div>
</div>
}

Answer 1

A repeat and edit of my comment on the original question:
Are you wanting full width on mobile and centered on desktop? I do this quite often with something like:
.small-12.large-6.large-centered.columns
or you can do something like
.small-10.small-centered.large-6.columns
You don't need the large-centered because it inherits it from the small-centered class. If you want the larger grid to not be centered, use .large-uncentered