Always work in the environment that your final render is going to live in.
If you work in SD and then crank up the frame size at the end, your scaling settings will be off.
Do what your heart tells you and dial back the render settings in the viewer window. Turn off shadows, reflections and depth of field. You can also set your resolution in the view window to half or third.
Andrew Wilson
WestView Digital Video & Design
http://www.westviewdigital.com