Я изменил свою панель навигации на панель навигации полной ширины, и после этого мои входные div и ul с иконками изменили ширину. Не могу исправить, что много пробовал, но я не знаю, как решить эту проблему. Я использую контейнер для центрирования этого содержимого на панели навигации, возможно, в этом причина.
<link rel = "stylesheet" href = "https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css">
<script src = "https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script src = "https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js"></script>
<script src = "https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js"></script>
<link rel = "stylesheet" href = "https://use.fontawesome.com/releases/v5.1.0/css/all.css" integrity = "sha384-lKuwvrZot6UHsBSfcMvOkWwlCMgc0TaWr+30HWe3a4ltaBwTZhyTEggF5tJv8tbt" crossorigin = "anonymous">
<nav class = "navbar-nav navbar-expand-sm fixed-top">
<div class = "container">
<a class = "navbar-brand"><img src = "path1610.png"></a>
<div class = "col-8 col-sm-6 col-md-7 col-lg-5 col-xl-5">
<form class = "navbar-form">
<div class = "input-group">
<input type = "text" class = "form-control" placeholder = "Let's rock!">
<div class = "input-group-append">
<button class = "btn btn-outline-primary"><i class = "fas fa-search"></i></button>
</div>
</div>
</form>
</div>
<button class = "navbar-toggler" type = "button" data-toggle = "collapse" data-target = "#navbarHidden" aria-controls = "navbarHidden" aria-expanded = "false" aria-label = "Toggle navigation"><span class = "navbar-toggler-icon"></span></button>
<div class = "collapse navbar-collapse" id = "navbarHidden">
<ul class = "navbar-nav ml-auto">
<li class = "nav-item">
<a class = "nav-link" href = "#">
<span class = "fas fa-user fa-2x"></span>
</a>
</li>
<li class = "nav-item">
<a class = "nav-link">
<span class = "fas fa-bell fa-2x"></span>
</a>
</li>
<li class = "nav-item">
<a class = "nav-link" href = "#">
<span class = "fas fa-fire fa-2x"></span>
</a>
</li>
<li class = "nav-item">
<a class = "nav-link" href = "#">
<span class = "fas fa-cog fa-2x"></span>
</a>
</li>
<li class = "nav-item">
<a class = "nav-link" href = "#">
<span class = "fas fa-comments fa-2x">
</span>
</a>
</li>
</ul>
</div>
</div>
</nav>





Просто изменил внешний класс навигации с navbar-nav на navbar и изменил container на container-fluid для полной ширины.
<link rel = "stylesheet" href = "https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/css/bootstrap.min.css">
<script src = "https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script src = "https://cdnjs.cloudflare.com/ajax/libs/popper.js/1.14.3/umd/popper.min.js"></script>
<script src = "https://stackpath.bootstrapcdn.com/bootstrap/4.1.1/js/bootstrap.min.js"></script>
<link rel = "stylesheet" href = "https://use.fontawesome.com/releases/v5.1.0/css/all.css" integrity = "sha384-lKuwvrZot6UHsBSfcMvOkWwlCMgc0TaWr+30HWe3a4ltaBwTZhyTEggF5tJv8tbt" crossorigin = "anonymous">
<nav class = "navbar navbar-expand-sm fixed-top">
<div class = "container-fluid">
<a class = "navbar-brand"><img src = "path1610.png"></a>
<button class = "navbar-toggler" type = "button" data-toggle = "collapse" data-target = "#navbarHidden" aria-controls = "navbarHidden" aria-expanded = "false" aria-label = "Toggle navigation"><span class = "navbar-toggler-icon"></span>
</button>
<div class = "col-8 col-sm-6 col-md-7 col-lg-5 col-xl-5">
<form class = "navbar-form">
<div class = "input-group">
<input type = "text" class = "form-control" placeholder = "Let's rock!">
<div class = "input-group-append">
<button class = "btn btn-outline-primary"><i class = "fas fa-search"></i></button>
</div>
</div>
</form>
</div>
<div class = "collapse navbar-collapse" id = "navbarHidden">
<ul class = "navbar-nav ml-auto">
<li class = "nav-item">
<a class = "nav-link" href = "#">
<span class = "fas fa-user fa-2x"></span>
</a>
</li>
<li class = "nav-item">
<a class = "nav-link">
<span class = "fas fa-bell fa-2x"></span>
</a>
</li>
<li class = "nav-item">
<a class = "nav-link" href = "#">
<span class = "fas fa-fire fa-2x"></span>
</a>
</li>
<li class = "nav-item">
<a class = "nav-link" href = "#">
<span class = "fas fa-cog fa-2x"></span>
</a>
</li>
<li class = "nav-item">
<a class = "nav-link" href = "#">
<span class = "fas fa-comments fa-2x">
</span>
</a>
</li>
</ul>
</div>
</div>
</nav>