youv'e got to keep in mind there is only 2 ways of making a larger image ( apart from re scanning an image )
one is to pixel stretch ( not recomended ) and the other is to pixel double ( a better method )
there is various dedicated software for pixel doubling that do it inteligently photoshop does a pretty good job
here's a page that will give you more info on the subject
http://graphicssoft.about.com/od/inc...n_Software.htm