Thunderbolt 27" resolution
The display is crispest at it's native resolution of 2560 X 1440. I would suggest adjusting the text size in each app. This is a bit of a pain because it seems that some apps render the same font/size combo differently. Most apps seem to have font/size adjustments with some, like Mail.app, having adjustments for each of the major control areas (mailboxes, summary, mail display).

There are third party apps like SwitchRes that may show you intermediate steps. When I run a monitor at a non-native resolution I get a headache in about 5 minutes. The only exception to this is on my Macbook Pro Retina running the "More Space" (1920X1200) setting in the Display preference panel.