It's OK to take liquids on aircraft - provided you bought them airside. You just can't take them through security. For instance, you can buy a bottle of water airside and take that on the plane. The size doesn't matter.
I always take an empty plastic 1 litre sports water bottle through security. When I get airside I then ask one of the outlets if they'll fill it with cold water for me and they are all happy to do that, especially if I'm buying a couple of teas. I've then got water to drink on the plane.