Hi! Ok I'll explain the way i resize my works:
1. First of all,this is how i have defined the canvas proportions.
2. I use SAI to do all the work I'm not sure if you can resize the image (canvas) in the same program, so I export the whole work when is finished to Photoshop and just choose this option (sorry my PS is in spanish) "Save for web".
3. Next i just "resize it" from this box
=> to this
4.Also i've noticed when i export from SAI to Photoshop the whole color features change too! ;3; making them more umm colorish/brilliant (? like this
Anyway I don't know if I'm doing it correctly because i have noticed that when I upload the resized image to Imgur (i use it to store my works) the quality is good there but when it "upload/post" the image here the quality goes loweeer (and i hate it) ;3; I don't know if it's normal or what but please heeeelp me >_<
Also forgive me if i can't explain properly ( my english is not very good e.e)