It is not necessary to chop apples so small. Any size that you prefer would be fine. In the original recipe, the apples are sliced into 3-4 mm pieces. Also, you can grate your apples. Everything depends on what texture of strudel you want. If you want a soft filling, then chop the apples small. If you want to have a harder filling and chew your apples, then you can make bigger slices.