It's not easy to find high-definition extra-large Mario or Sonic wallpapers at the moment, so to help out if you are having problems finding some here are two 1600 x 1200 pixels sized ones (shown here at 400 x 300 pixels - click on the images, or the links below each image, to go to the relevant full-size pages for them):
1600 x 1200 pixels version
1600 x 1200 pixels version