The short answer is: there's not a whole lot you can do about it.
If you want your game to scale to the available area and the available area becomes smaller, logically, your game will scale.
You could try to scale your game back to the size it was before scaling down, but the problem you get then is that your textbox could be obscured by the pop up keyboard.
Another workaround would be to not use the system keyboard but create your own keyboard in your project.