Yosemite: Safari's window is too wide
Solution 1:
You can enable "Develop" menu in Preferences > Advanced.
Then you can select choose the User Agent to be iPhone or whatever.
And then resize the window appropriately:
Edit: This doesn't get around the minimum width for the window. Here is a workaround. Go to View > Show Sidebar. This pulls out the sidebar, which can be resized making the viewport smaller.