I don't think there's a way to do what you want. Maybe you could manually figure out how big each tool tip box should be. Then ... (let's assume you have a function that creates a tool tip with a given text) you would have to pass the desired size as a parameter too.
Maybe just create a status bar. You could put a very long text box across the bottom of the screen. Then you could have the tool tip text go to the status bar.
I know this isn't exactly what you wanted, but it may be the next best (doable) thing.
But it seems like it would be difficult for the user to automatically determine how big the text box should be just based on the message text. (Unless it were a feature built in to Construct that shrunk the text box to fit the message.)