Just as an example:
The original sketch is about 700 by 700
Resize the image pixels, not the canvas by percentage to about 1000 by 1000
then do the lineart
the coloring etc
and when you size down at the end by percentage and get back to the 700 by 700
it ends up how mine is
And once you resize down, it will become blurred as long as you do not zoom in or try to enlarge again after downsizing. Once you're done, you're done. That's why I have a PSD file with the 1000 by 1000 and a PNG of the 700 by 700