It's quite simple:
make object plain 1 , 64 , 64
lock object on 1 : rem locks it to the screen, position is now relative to the camera
position object 1 , 0.5 , 0.5 , 400
set object filter 1 , 0 : rem stops the object from being all blurry
The above code will make the plain object exactly 64x64 pixels, but this is only the case if your resolution is
640x480. For higher resolutions, you'll need to scale the object down proportionally. The distance on the Z axis should always be 400.
Also note that when using 3D objects, the pixels are rounded centered, so you actually have to offset the object by 0.5 pixels in each direction or you'll get strange distortions (as shown in the code snippet).
TheComet