Making Resized HD/Hi-Res Sprites
For this tutorial, I am using a basic standing sprite of Sonic from Sonic the Hedgehog 3 for the Sega Genesis.

When resizing sprites, it doesn't matter how you do it, as long as you can make out what it looks like when it's resized. Yes, this means you can drag the corners of the sprite and manually resize it yourself, as long as you can tell what's what and organize your sprite correctly.



Step 1:
Resize the sprite you're using. For this sprite, I chose to resize to 2x size.
Map out the palette, and then color over the entire sprite with a color that stands out opposite from the background.
(I used yellow because it helps me focus more on the edges because it doesn't blend in with the background at all. You don't have to use the same color, though.)

Step 2:
Once you resize your sprite, smoothen the edges. Use the original sprite as a reference if you need to. Remember that your sample sprite is a reference; don't rely on it throughout the whole process. Don't try to nail it shape-by-shape, because chances are it'll end up looking really ugly and awkward.

Step 3:
Once you've smoothened your edges, start mapping out your colors. Again, use your reference. Don't stress over details just yet, since all you're doing is mapping out the sprite.

Step 4:
Begin the shading process. Use only the colors you're provided. You have freedom at this point, so don't focus on nailing it down to the last pixel; add your own flavor to it if you want, and feel free to shade things a bit differently. Remember to be patient, or your sprite won't turn out the way you want it to at all.

You're pretty much done here.

Step 5 (Optional):
If you wanna add a "true" hi-res effect, throw in what I like to call "in-between" colors. Use these to AA your edges, to add that nice smoothing effect you see in most larger sprites. You can also use in-between colors to add additional outlines and details.