My 2 cents worth:
Think about physical resolution first, and monitor size second.
Most existing screens sets are set up for 1024x768 physical resolution.
At a given resolution, the larger the physical size, the larger the images you look at (this is where the age of eyes consideration comes into play).
There are also screen sets designed for 1280x1024. The installed base of mach users tend to have 1024x768 displays.
However, new 1024x768 monitors are becoming rare - 1280x1024 or wide TV (16:9 instead of 4:3 aspect ratios) are now as cheap as 1024x768.
The new screen sets for Mach3 V4 support both 1024x768 and 1280x1024.
The 1280x1024 screens have additional controls on some pages and are visually "less crowded".
I happen to prefer that the monitor physical aspect ratio match the screen image aspect ratio (I like my circles to be round and not appear as ellipses), so my personal preference is for 1280x1024 on a 17" monitor - just happens to be the best combination for me of the available choices.
Dave