NotifyIcon.BalloonTipText max length = 255 characters
NotifyIcon.BalloonTipTitle max length = 63 characters
Source: Trial and error.
Good luck finding these specs anywhere. Seems to be one of the lesser documented items in the Windows API (or it requires a very specific search term). Funnier still is you won’t get any compiler errors in Visual Studio 2005/2008 if your string is longer than the above mentioned lengths – you’ll just have to wait for your balloon tip to get cut off when it’s displayed.
Stephen Sulzberger's Blog 