Ok, my photoshop is in french, so maybe I won't use the right words for the commands, but you can downsize your image by using the resize command from the image menu, just as you would resize any image. There you have different resizing options when ticking the "resample" box . Bicubic is on by default, but from the drop down list you can change this method to bilinear. Bicubic sharpens the image while resizing, and bilinear smoothes out the image which is what you are looking for if your render isn't anti aliased. In CS2, you also have "smooth bicubic" I haven't tried it, maybe it is as good as bilinear.
Again, the translation may not be accurate.