This might seem a little elementary, but I can't to find out what size of padlock is required for a typical hostel locker. Google seems to return only people with stories of buying padlocks that were too big, which isn't much help.
My search did lead to a 'Pacsafe RetractaSafe', but the security of this seems even more dubious than that of a padlock, which would at least require a thief to carry a conspicuous bolt cutter.