Wow an amazing display of sketches Well done
.They look fantastic.
For posting the image smaller i'd suggest just scaling the image down using a photo editing program.
Regarding the sketching its great but what you might like to do is if you have a photo editing program like photoshop or gimp then i'd suggest trying to make the stars look rounder and maybe a gausian blur to produce that subtle glow from galaxies and show how faint they are.
Great work
Orestis