This study compared 26-Inch MultiSync widescreen (MS LCD2690 at 1920x1200 resolution), 24-inch MultiSync widescreen (MS LCD2470WNX at 1920x1200 resolution) and 20-inch MultiSync traditional format (MS LCD2070NX at 1600x1200 resolution) monitors.
There was no 30" monitor involved in either of the studies, nor was there any monitor with a resolution above 1920x1200. The slide with a graph showing a drop-off from 26" to 30" is an estimate based on the 4 configurations they tested (the 3 listed above and dual 20" standard monitors).
As far as I can tell the findings of the study were:
1. The 24" and 26" widescreens outperformed the dual 20" standards on a text editing task. 2. The dual 20" standards outperformed the single widescreens on a spreadsheet task, but by a lesser margin. 3. The 26" widescreen significantly outperformed the 24" widescreen on the text editing task at the same resolution.
How do you conclude from this that 2500x1400 is the optimal resolution for the vast majority of people?