Should I buy smaller clothes or large sizes clothes?

Perhaps you need to buy both. It's always best to buy clothes for your current size, and then buy a couple of pieces for the next size up if you know you are gaining weight/growing taller and such. On the other hand if you are trying to lose weight, buy a couple of pieces in smaller sizes as your goal pieces, and then eat less, but exercise more, so you can fit into those clothes sooner.