If you wear glasses and get eyestrain reading a small screen, then try taking the glasses off. Without my glasses, I can read my T|X's screen for as long as I want without any problems.
For a desktop computer, the best advice I can give is relatively large fonts (10-12 px minimum, depending on your resolution), and
sit back. It's much easier to use a computer screen for a long time if you're more than a foot away from it (as I type this, I'm leaning back into a chair, eyes about a metre away from the screen).
And if you're still using a CRT monitor, dump it, buy a LCD, and cite health reasons if anyone asks why.