While many apps provide zoom for content they present, I am yet to find an app that allows increasing font size in its UI elements (Though I think OSX should solve this problem in some standard way, not individual apps).

Regarding screen size - the fonts are OK on my MBP too, but 27" is another story - you have to sit farther from such a huge screen. iCal and many other apps on my parents' iMac 24" are pretty much unusable because of tiny font sizes.