The weather and the crowds can affect if the lockers are taken quickly, but we have never not been able to rent one (and we do so most every day).
We revisit the locker a couple of times per day to either put in or take out extra clothing. (You can open the locker as many times as you want without having to "add" any money.)
There is a locker station near the front (after you're inside the gates) of every park.
Its $7.00 per day (or so) to rent, but you get $2.00 BACK when you return the key.
If you park-hop, just keep your first locker's receipt and you can get another locker at the next (and the next) theme park (that SAME DAY) for no extra charge (except another refundable $2 key deposit.)
You pay a CM a locker fee and key-deposit, CM hands you a locker key (with a number).
You go to that numbered locker and open with the key, put in your item(s) close and lock the door, take the key.
When leaving the park, open your locker with key, remove item(s), close & lock door, take key to locker CM, receive your $2.00 key-deposit return.
There are 2 sizes of lockers at Epcot and the Studios (first-come, first served if you specifically want a "bigger" locker).
There is only one size of locker available at DAK (moderate size) and MK (pretty good size).
Lockers at MK have just been moved to a new location and are all brand new this year.
I'd say big enough for an avarage backpack and a few extra jackets, etc.
The ones at DAK are the hardest for us to "jam" our backpack into, but it still fits.
