CAD is great, and there are some OK free and $50 ones, and areally nice range in TurboCAD...but those are vector programs that won't help too much, as you are working in an image or bitmap (specifically jpg. There are some trial packages that will roughly trace a jpg to CAD, but require a lot of cleanup.
I think a free image editor (photo image) program would do the trick for you. As Simon says, though, you need to check resolution and image size, and then it's just VERY simple math to scale. For example lets say in the hardcopy world you had a plan that was at 1:100 and you want to make it 1:72. Enlarge the image 138.88%. (100 / 72). A better way to "see" it is, divide your target by your present size. If a sailor is 6foot tall in 1:1, what would he be in 1:72... 6x12inches = 72 inches divided by 72 (scale) = 1 inch tall (In metric that's 1828mm / 72 = 25.3mm). Or in 1:48 6ft = 1.5 inches (38.1mm).