You don't! A pixel is a pixel, there's no such thing as a "8-bit pixel". If you design gfx for a game that will run at a lower resolution or retro-style then it should be scaled by code in realtime.

But what you probably mean is "how do I make that old game look the same size on my new computer"...well, scalefactor = new resolution / old resolution (preferably rounded down to a whole integer).

Edited by DawnBringer - 09 January 2012 at 6:09pm

The simplest is to edit it at its native size (here: 128x64), and only scale it up 400% when you need to export it : for web, wallpaper, etc.
If you didn't keep the orginal as a separate file, you can reverse the operation by rescaling this one 25%.

All good painting programs allow zooming anyway, so you can watch the pixels at any size while you're drawing.
On PixelJoint Gallery, pieces should be submitted at smallest size : there are some + - buttons to zoom at well.
And in the forum, clicking any linked image will zoom it in. Use control-click to zoom back out.

...Use any painting painting program that allows it. Depending on the program, it can be called stretch, resize, resample, transform...
Even MSPaint has it.

If the program lets you choose a sampling method, pick "nearest"; all other methods (bilinear filtering, interpolation,..) will cause a smoothing that's unwanted in this case.