From: Daniel Piechnick
Hi Sean. If you really have to make things exact, the way I would do it (although it's clunky) is to create a new image of the desired size. Press Ctrl-A to select it all, then drag the selection onto your main image. Then, when you use the Slice Tool, you can just follow the boundaries of the selection, and use the selection like a portable ruler. Ask yourself if you really need to make things exact though. (I described the rest of the Slice Tool process in an earlier post.)
