How do I customize the ember-bootstrap form.element so that the input uses a specific column size?

I'm using ember-bootstrap and the following element:

<form.element @controlType="text" @label="First Name:" @property="firstName" />

which produces the following HTML

<div class="form-group row">
  <label class="col-sm-4 col-form-label" for="ember198-field">First Name</label>

  <div class="col-sm-10">
    <input id="ember198-field" class="form-control" type="text">
  </div>
</div>

How do I go about customizing the col-sm-10 in the enclosing <div> for the input element?