Basically, you need the next:

while( is_overflow_in_the_second_div() ) {
    set_smaller_font();
}

So, you should check questions with

• how to detect scrolbars
• how to detect overflow

and like, e.g.:

• is there a way to fire a event when vertical overflow is detected in a div?
• Detecting whether there's overflow or not WITHOUT javascript
• How to detect overflow in div element?
• Check whether HTML element has scrollbars
• CSS - Apply Another Style if Content Overflows to New Line
• How to determine using JavaScript if HTML element has overflowing content

maybe many others.

Maybe you don't searched the questions for the right keyword, but this is probably an duplicate. :)