The maximum length of a standard tooltip is 80 characters (this is not a limitation of just zenon, but any application that uses Microsoft's standard tooltip function).
There are also multi-line tooltips that do support more characters, but currently zenon uses the standard tooltips for the elements in the runtime.
I don't know what this would involve if multi-line tooltips would need to be implemented in zenon, instead of the standard tooltips, or if this would technically be an option.
I recommend you to suggest this enhancement to your local COPA-DATA partner, with some more information why this is helpful.
Have a look at this thread where an example project has been posted by tsragland, using VSTA to display longer tooltips. (you need 6.51 + a current build)