The first picture native size was 870x1000. After I finished and resized to the final size (450 px wide), I found it too coarse and unrefined.
So I went back to the original file, scaled it up of 1.4, and retraced it (with the same brush size, which I think it was 5 or 6 pixels). Here below you see the final results.
(I changed also the color palette, but that is not the main point).