I never really looked closely at the lockers in lounge 52, having never had the need to use them. But they should fit a carry on backpack without difficulty.

To exit it's not the same elevator that takes you down to the lounge that also goes to passport control/exit, but the exit is nearby.

Coming back in security control and passport control are directly after each other on the upper level (level 2 on the map). You'll come out in what schiphol calls "lounge 2" on the mezzanine near McDonalds, from where it's a short walk back to the crown lounge.