There's no "real way" to do this, however what I will usually do is start an image at a size that fits in the main window, do my image, then size it up before I do my final render keeping the aspect ratio the same will not effect your scene, in other words If I start at the default canvas size and then size it up for a final render, nothing is lost or added. You can also stop a render in progress and while the partial render is up on the screen use command and the + or - keys to zoom out and then hit resume render to see it all render out. On a PC I think it's alt and the + or - keys to zoom it in or out.

Do atleast one thing a day that scares the hell outta ya!!