It's styled to occupy 100% width, so we need to change it's parent’s width (which has float:right)

In the Admin Panel go to [ Appearance ] -> [ Edit this Layout ] and paste …

form.navbar-form.navbar-right {
  width:120px;
}

@media ( max-width : 768px ) {
  form.navbar-form.navbar-right {
    width:auto;
  }  
}

… into the editor area on the left. 120px is the new width. You name it.

Preview / Save

The @media rule unsets the width for screens narrower than 768px (that's the breakpoint where the menu becomes button-triggered and full-width)