Get the bottom right corner of a textbox

Situation:
I need to be able to detect when the user has filled up an input textbox. Normally, if the user keeps typing the textbox will just scroll down a line and the user can keep typing.

I need the user input to stop and perform some other functions once the user has reached the original hieght of the textbox.

Setting a character limit is not an option as the font sizes will be changing.

Here’s a little screen grab of what I’m talking about

So what and is there a function that will allow me to do this?

Thanks
-D