Hope this thread's still monitored...
Anyway, Ich reported in this thread
http://forum.shrapnelgames.com/showthread.php?t=44045
...that the "parser" can handle 2500 char descriptions for nations and units. However, the actual in-game limit for mod #descr values appears to be 999 chars (some in-game nations, e.g. LA Caelum exceed this, presumably because they are not read in like mods.) It would be nice to allow the full available length for descriptive text.