Encourage the children to use increasingly sophisticated language to describe and compare what they see. They may begin to identify the different kinds of triangles they see and to make suggestions about the size comparisons between them using numbers and fractions. Notions of rotation may also be expressed.
Some children may need to stick to using colour to identify the triangles and compare them more generally in size looking for the bigger and smaller ones. They may also be able to count the triangles at different stages if you freeze the frames for them.