Fancy Dress and Halloween Costumes to fit Teenagers. A brilliant range of boys and girls teen costumes. Boys teen costumes come in two sizes 11-12 and 13-14 (the next size up would be men's small), girls teen costumes come in ladies XS size, which is approx a UK 6-8. There is a wide selection of costumes available. Have a browse and see what takes your fancy. Get great prices and fast delivery when you buy online from XS-Stock!