<%= options_for_select(
  ['Choose a size', 'small', 'medium', 'large'],
  nil,     # selected value
  'medium' # disabled value
) %>
 
<!-- Gives you -->
 
<option value="Choose a size">Please choose a size</option>
<option value="s">small</option>
<option value="m" disabled="disabled">medium</option>
<option value="l">large</option>