I'm going to start out using this pattern I scanned from Moden: 1840-1890. Unfortunately, it's in Danish. Fortunately, that isn't really important.
You're going to want to make your canvas really large in order to give you lots of room in case the finished product is bigger than you expect. I don't expect the bertha's going to be anything over 50" x 50", so that's what I set it as.
Select the part of the pattern you're working with and select it with the square marquee tool. Go to Edit>Free Transform. This is only if you don't know the exact scale, i.e. 1/8 of the original scale. If you do know the exact amount you need to scale it up by, skip ahead.
|Meh, close enough.|
So now that I'm in Free Transform I can grab my picture and make it smaller or larger. I use the ruler tools on the side as a guide - I know one box is a bit less than two inches so I try to make it as close to that as possible.
If you already know the magnification, just type it into those boxes once you're in free transform (as in, if the pattern was 1/8 scale, make the height and width 800.00%.)
Now that you have your resized pattern, it's time to split it up and print it out (unles you have a kinko's size printer at home, that is.)
Feel free to ask questions! I'm happy to procrastinate about not wanting to sew my stays. After 100 something boning channels I'm less than enthused.