I put my luggage in a Jumbo locker in the hidden locker/picnic area outside the two parks. It was $15, and I could have fit myself AND my bag+carryong in there. It was way way too big.
I don't think the bigger lockers are inside the parks...only smaller ones for a smaller cooler with foods, bags, that sort of thing.
If you want the bigger ones...they are outside of bag check on the DTD side. If you were walking with DTD at your back and the bag check facing you, you would take a left after you pass the fountain (also on your left), and go towards some tall "wind block" type of trees. The monorail goes right over the area, if memory serves. There are concrete picnic tables and benches, and opposite the entrance there are lockers. Like I said, the Jumbo was far far too big for my bag (which I use as a checked bag but the dimensions would allow it to be used as a big carry-on for all airlines) and my carryon. So I would get at least one size smaller than the Jumbo.