actually most of the pageant either local or not, it is not really necessary to wear short dress, short dress is only necessary if the talent portion number of your daughter requires for it. Pageant required long dress but you have to make sure that it is just in the right length so she will not have problem in walking. I suggest you cut the length according to what is comfortable for her height and to walk with the dress too. I can reckon that you ask your daughter to wear the shoes that she will going to wear with the dress and from that you take measure of how long she wanted it.
I hope this answer help you.. thanks for asking