Dave:
The way I have used resizing in the past is to enlarge the image by a small percentage. I don't remember the exact percentage I used, but 10% probably sounds about right. I then scale the image up again by this amount. I continue to do this until I reach the final size I need. As a default I use the BiCubic method.
Again, there are limits. A two megapixel image is not going to scale up very far without it showing signs of falling apart. A six megapixel image is going to have lots of rich details that will hold together through multiple scaling sets. The more information contained in an image the better it will respond to small steady increases in size.
It is important to do it as a TIFF or Photoshop format. You do not want any compression algorithms playing with your pixels. The best way to see how the progression is going is to keep an original copy and compare it at 100% of actual size to the upscaled copy viewed at 100%. How do the pixels in the same areas compare for sharpness, color, and contrast?
Tom
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