For a project I am working on, I use the following in my layout _footer partial. It gives me two small links at the bottom of the window that when clicked, open up either a portrait or landscape style browser window with a fixed small size resembling a mobile device. Note the "m." in the URL for the window.open as I use a "m." domain prefix to denote the mobile version of the site. You may have a different way of indicating that you want the 'mobile' version of your site.
<% if ENV["RAILS_ENV"] == "development" -%>
<script>
function mobilePortraitPopup()
{
window.open('<%= request.protocol + "m." + request.host_with_port %>', 'Mobile Portrait', 'menubar=no,scrollbars=yes,width=320,height=480,top=50,left=300,scrolling=yes,toolbar=yes,resizable=no')
}
function mobileLandscapePopup()
{
window.open('<%= request.protocol + "m." + request.host_with_port %>', 'Mobile Lanscape', 'menubar=no,scrollbars=yes,width=480,height=320,top=80,left=330,scrolling=yes,toolbar=yes,resizable=no')
}
</script>
<p class="small quiet">
<a href="javascript:mobilePortraitPopup('')">mobile portrait</a> |
<a href="javascript:mobileLandscapePopup('')">mobile landscape</a>
</p>
<% end -%>