You haven't shown the code that changes the font-size; does it set the cookie at the same time?
You aren't setting an expiry for the cookie. This means that it will expire when the browser closes. I cannot recall whether this also applies if you close the tab or move to a different page - would need to check this.
You are using unescape() when reading the cookie but your are not storing it using escape(). But these two functions are deprecated and you should use encode.. and decodeURIComponent() instead.
"I'm here to save your life. But if I'm going to do that, I'll need total uninanonynymity
." Me Myself & Irene
Validate your HTML