I'm debating whether to get a locker or not. It came in handy last year although I wasn't camping. Still, it would be nice to exchange sunblock for glowy shit in the evening without having to go through the security line in the evening.
There's no need for a large locker. I've gone with the smallest size the past two years and it was always more than big enough. I always store my backpack and sweatshirt in mine, and those literally only took up about 1/5 of the entire space.