<div class="row">
  <h2>Sign up</h2>

  <%= form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
    <%= devise_error_messages! %>

    <div class="field">
      <%= f.label :first_name %><br />
      <%= f.text_field :first_name, autofocus: true %>
    </div>

    <div class="field">
      <%= f.label :last_name %><br />
      <%= f.text_field :last_name %>
    </div>

    <div class="field">
      <%= f.label :username %><br />
      <%= f.text_field :username %>
    </div>

    <div class="field">
      <%= f.label :email %><br />
      <%= f.email_field :email %>
    </div>

    <div class="field">
      <%= f.label :password %>
      <% if @validatable %>
      <em>(<%= @minimum_password_length %> characters minimum)</em>
      <% end %><br />
      <%= f.password_field :password, autocomplete: "off" %>
    </div>

    <div class="field">
      <%= f.label :password_confirmation %><br />
      <%= f.password_field :password_confirmation, autocomplete: "off" %>
    </div>

    <div class="actions">
      <%= f.submit "Sign up", class: 'button save' %>
    </div>
  <% end %>

  <%= render "devise/shared/links" %>
</div>

<div class="row">
  <h4>Register</h4>

  <%= form_for(resource, as: resource_name, url: registration_path(resource_name)) do |f| %>
    <%= devise_error_messages! %>

    <div class="field large-8 small-6">
      <%= f.label :first_name %><br />
      <%= f.text_field :first_name, autofocus: true %>
    </div>

    <div class="field large-8 small-6">
      <%= f.label :last_name %><br />
      <%= f.text_field :last_name %>
    </div>

    <div class="field large-8 small-6">
      <%= f.label :username %><br />
      <%= f.text_field :username %>
    </div>

    <div class="field large-8 small-6">
      <%= f.label :email %><br />
      <%= f.email_field :email %>
    </div>

    <div class="field large-8 small-6">
      <%= f.label :password %>
      <% if @validatable %>
      <em>(<%= @minimum_password_length %> characters minimum)</em>
      <% end %><br />
      <%= f.password_field :password, autocomplete: "off" %>
    </div>

    <div class="field large-8 small-6">
      <%= f.label :password_confirmation %><br />
      <%= f.password_field :password_confirmation, autocomplete: "off" %>
    </div>

    <div class="actions">
      <%= f.submit "Sign up", class: 'button save' %>
    </div>
  <% end %>

  <%= render "devise/shared/links" %>
</div>

Reduce registration form field width
Reduce registration form field width
