How to make a Div Fixed Size html 5

[dano]

I made a site so when you click the button 30px it changes the font inside the \[code\]div\[/code\].I have a \[code\]ul\[/code\] inside it with the bottons. When I change the size of font the \[code\]div\[/code\] expands and the nav buttons move with it as well.How do i make it so it stays fixed but the fonts can still change.