Okay kiddo, have you ever heard of clothes that are too big or too small for you? That's kind of like what cold sizing is, but for pipes and equipment that are used in really cold places like in the Arctic or Antarctica.
In really cold places, things can freeze and stop working. So, to make sure that everything keeps working properly, they use something called cold sizing. This means that they make the pipes and equipment a little bit bigger than they normally would be so that they don't freeze or break in the really cold temperatures.
It's kind of like wearing a bigger jacket when it's really cold outside so that you stay warm and don't get sick. Just like how you need to wear a bigger jacket to stay warm, the pipes and equipment need to be bigger to work properly in the really cold temperatures.
Does that make sense to you?