If you have an LCD, it's always best to set your operating system to drive the display at its native resolution. In my case, this is 1600x1200. If you find font sizes too small, you can increase the DPI in the advanced settings of Windows' display properties, but this causes issues with some programs.
The best solution is to check around before you buy, and purchase an LCD with a pixel size that is acceptable to your eyes. For example, 19" and 26" LCDs have larger pixels than their 17" and 24" cousins (both respective pairs have the same number of pixels, so the larger screen's pixels are larger).