The poster is right -- don't write on a whiteboard. Instead, bring your laptop, make the font bigger on your favorite editor, and write in that. I've gone through many interviews like this, and only one interviewer insisted I use the whiteboard. And I'm pretty sure that was largely out of wanting to be fair to other candidates who didn't think to bring and use their laptops.
Since you will presumably be writing code on a computer when working at the company, why not write code on a computer when interviewing with one?
Since you will presumably be writing code on a computer when working at the company, why not write code on a computer when interviewing with one?