Filling the whole screen in 800x600 (windowed mode)

["M.C." fenda@xxxxxxxx]

I am working in a font for windowed mode (800x600) and I have a doubt I
need to solve. In the 800x600 screens I have at hand to test, the matrix
that fills the whole screen is not the same under different op sys.
Under W95, the font that fills the whole screen is 10x20 pixels; under
NT, it's 10x22. So, I need to make a choice. Any information is
mandatory to go further and, believe me, it's not a simple task, because
that small matrix hardly lets me combine design and readability.
Manuel Castelao