Go to the Filters page.
If you want to keep the screen size but accept a bit blurry display, then just move the slider below "Point/Bilinear" to the far right.
If you prefer a crystal clear but blocky display, keep the slider on the left but change the scale factor above it from "default" to "no scaling" and change Horiz./Ver. size both to 2x. To see the entire screen now, you have to set the display to full-window or full screen. For the latter choose "native" resolution.
