If you're planning on using the second monitor mostly for games, or TV, then a big TV would be a grand plan (nothing quite like some COD4 action on the big screen!)
However, if you're going to be doing a lot of work (web design, word processing, graphics etc.) on it, then I'd probably go for a big widescreen (22-24") - you'll still get a fantastic gaming picture on it, but it'll be easier on the eyes when doing work.
I've got a 21" and 24" widescreen dual setup. It's a lovely sight