I don't think switching to 250ml cans would save aluminium, because you'd presumably need more aluminium to house the same amount of drink with smaller cans (i.e. smaller cans will have worse volume to surface area ratio). You'd buy an 8 pack of 250ml instead of a 6 pack of 330ml and waste more aluminium. Presumably the manufacturing cost, energy usage etc per litre is going to be higher with smaller cans as well.
As for the quantity I've been disappointed at small cans running out more often then I've felt I've got excess drink in a 330ml can.