Increasing or decreasing file size: Resampling
As we mentioned in the Cropping for better composition section, resampling is the process of adding or subtracting pixels to any file to make it larger or smaller.
Now, you might think, if this is the case, then your dreams of producing very high-resolution files have just been answered. But there's a catch. Resampling is a mathematical algorithm that can be used to upsample or downsample files effectively, provided that the original is of the highest quality.
In practice, this means that if you try to resample a 1 Mp file so it can be printed as a poster, it's not going to look very sharp. But if you start with a lot more image data—let's say, a 24 Mp file—this can be resampled to 30, 40, or 50 Mp with almost no loss of quality. It could go as high as 200 Mp before it really looks soft.
Further sharpening after the resampling process will help to reverse the softening that adding extra pixels might...