They get the width from the select element at boot:
select {
width: 220px;
border: 1px solid #cccccc;
background-color: #ffffff;
}
To fix, add your own at the bottom of the overrides file:
select.date-select {
width:auto;
border: 1px solid #cccccc;
background-color: #ffffff;
}
The next problem is applying the class to date_select elements. If you just throw: class => "date-select" there, Rails does not recognize that you are trying to use both parameters and html_options. Instead, you should pass each as a hash:
<%= f.date_select :start_date, {:prompt => { :day => 'Select day', :month => 'Select month', :year => 'Select year' }}, {:class => 'date-select'} %>
date-select .