I think the description box has no actual limits, apart from only supporting 2 lines (by default).
So you can fill out the box as you want. (the 2 line limit can be circumvented with line-breaks which some plugins provide)
But the amount of characters that can actually be shown in-game is limited by the size of the description window (so basically your game resolution)
Good to know, that there are no limits, because I'm struggling with dilema how to handle descriptions, read them from text file or generate formatted text (MV's text tags) and paste it directly into description field.
Kind of sucks, I like long item descriptions, especially on unique stuff :3 Dragon Age sometimes have like a short stories on some of the gear I'd also like a description on parameters tbh but haven't found a way yet