Something which is often forgotten: bigger isn't always better.
I mean, it depends on how close you sit to your monitor. A wopping big ass LCD might be far worse for your eyes (rapid tiring from looking all over the place) than a smaller "normal" sized LCD. You also would see a lot more of the "pixels" if you're closer to the LCD than on a smaller LCD (with the same resolutions).
I would say 19" is minimum if you're like 1m away from your monitor. But 24" is way to big for many desk layouts though (sure it looks awesome, but it isn't very healthy and you will get tired quicker from staring at that big screen)!!
To test this distance from the screen yourself, do normal computer stuff and after an hour or so, freeze your head and body and take a ruler and measure the distance from your eyes to the screen.
The reason I suggest to do it after 1 hour of normal PC use is that this would be the most 'natural' position your body is in and the measurement would be far more correct than when you explicitly get behind your PC to measure the distance. Unconsciencly you would position yourself to a bigger distance from the screen than when you don't think about it (and the difference can be very big).
You can use a alarm clock to sound an alarm after 1 hour, or let your mom/friend/whoever come in after 1 hour and do the measurement.
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