That's why it is always nice to get in the habit of rendering to disk at high-res on all your images from the start, which is also an advantage if you want a personal print or if you plan on selling your art in the future. Not a Mac guy, but I would fair to guess you at least have a dual processor and you can take advantage of setting your render priority to high right before you render (Have to do it everytime, default is normal) and you can get much faster renders at the bigger size you want... and If you have alot of images to re-render, you'll save so much time!