Just to make sure you (and ryck) are clear about this: the (standard) 27" screen measurement refers to the diagonal of the actual screen, NOT the outside measurement of the entire monitor or iMac. Other critical measurements are
native resolution (in pixels) and pixel size (smaller with e.g., retina screens). For example, if the native resolution is the same for a given 27" and 30" monitor, the latter has slightly larger pixels. If the 27" has a slightly larger resolution, the screen real estate may in fact be larger than that of the 30", while pixels may still not be too small to cause vision problems.
In practice (see
Mactracker), the 27" and 30" screens only differ in the number of
vertical pixels, with the 27" having 10% fewer:
30" (29.7" viewable area) display (2005) resolutions: 1024 x 640, 1280 x 800, 1920 x 1200, 2048 x 1280, and 2560 x 1600
27" (27" viewable area) display (2011) resolutions: ---------------, 1280 x 720, 1920 x 1080, -------------------- 2560 x 1440
There are also differences in brightness, contrast and screen reflectivity between the two monitors. With screen reflectivity
* affecting the subjective perception of brightness and contrast, you may want to actually see a 27" screen to find out how it compares to your 30" monitor. You just might be pleasantly surprised.
*) Since the current iMacs have the front glass glued to the LCD, which markedly improves the screen, the 27" monitor may get this 'upgrade' as well (if it doesn't have it already).
