For General Use, a resolution of about 75dpi is quite comfortable.
This equates to:
1024 x 768 on a 17 inch monitor
and
1280 x 1024 on a 21 inch monitor.
For detailed CAD work, I sometimes change to 1600 x 1200 on a 21 inch monitor, which is about 95dpi.
There are official Standards for VDUs that give recommended character display heights.
Regards
John

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