There are two kinds wheels,one kind with total two wheels,can't climb the stairs,it is flashing wheels,the number 1,3,5,7,9,11,13,15,17,19,21 and 23 are two wheels style,

the other kind with six wheels,can climb the stairs,not flashing wheels,the number 2,4,6,8,10,12,14,16,18,20,22 and 24 are six wheels style.

Number 1,2,3 and 4 size about 37*32*18cm,normally suit for kindergarten
Number 5 to 18 size about 42*31*18cm,normally suit for grade 1-3
Number 19 to 24 size about 45*35*20cm,normally suit for grade 3-6

When you order,please kindly pay attention to the above message.