Safely enlarging digital art
You can always reduce digital art without sacrificing image quality. However, trying to enlarge art will usually result in poor image quality. That’s because you’re effectively reducing the DPI and the LPI of the image as you stretch it. If your original art has a high enough resolution, however, there’s a workaround. Here’s how to do it in Photoshop.
Select Image —> Image Size.
Place an X inside the box near “Constrain Proportions.”
Make sure there’s no X in the Resample Image box.
By typing a lower number in the resolution field, you’ll notice that the physical size of the file increases. For example, a 5 in x 7 in file at 600 DPI will enlarge to 10 in x 14 in at 300 DPI.
This presents a really good argument for using the highest-possible resolution on your master scan of a comic.
Those of you drawing digitally in a vector-based enviroment (Adoble Illustrator, CorelDraw, FreeHand, etc.), can reduce or enlarge without worying about these issues. Since your art is vector-based, it’s not constrained by resolution.