I just got a T60 with 15.4" WSXGA+ display. This is as high resolution as I am willing to go on this size. Any higher, my eyes will need to be uncomfortably close to the display to make out the words. On the other hand, I love the large desktop space I get.

To get an idea on how small the default text size will be, you can look at the screen's pixel density, usually in dots per inch (dpi) unit. This measurement is usually not readily available on manufacturers' page, but can be easily calculated from the resolution and size information available. There is even a handy calculator page: http://www.raydreams.com/prog/dpi.aspx

I have used all sorts of displays from 80dpi (15.4" XGA) to 200dpi (7.2" WXGA on Libretto), and found my comfort zone to be around 120dpi. For what it's worth, Windows' OS default is 96dpi.

Lenovo offers these 15" displays on T60 series:
15.4" WXGA 97dpi
15.4" WSXGA+ 130dpi
15.4" WUXGA 150dpi