Size CODE HEELP PLZ

Hi, i am doing a notepad program ...and i am trying do a button what when you click the text size goes to 10 or 20 or what i want...

BUT I TRYED EVERY THING BUT I CANT ..

Example..

Button Size

When you click , the textbox change the text size for something?

PLEASE HELP

Yeah its better...There no one can help me..
You can close this.. -.-'