One of the the things I have learned over the past while working in any graphics program is to work larger than the final output. If my final image is to be 800x600px... I am sure to be working at 1024x768 or a little larger for most projects.
Reason? It allows me to apply a little more detail to an image and manipulate it easier when I have more pixels to work with. This is especially helpful when making masks and when trying to add, improve, or bring out small details in an image. The more complex an image... the harder it is to make pixel based edits on it when its smaller.