For those who have been waiting to update their Windows Phone 7.x applications to take advantage of what Windows Phone 7.8 has to offer (New Live Tiles), the SDK including the new Emulators is now available.
To manage expectations: This is really about all this SDK Update gives you – ability to test new Live Tiles until the phone OS Updates start rolling out for actual devices.
Direct download link: http://www.microsoft.com/en-us/download/details.aspx?id=36474
Official Blog Post explaining how to leverage the New Live Tiles sizes in Windows Phone 7.8 SDK:
Get your Windows Phone 7.x Device Updated to 7.8 - How to Update to Windows Phone 7.8