An answer without 儿 should also be accepted.
Duo currently insists on "条" here but "件" in other questions with "裙子".
I think both should be accepted because 条 is a counter for long objects and 件 for all clothes in general.
Why not 这条裙子一点点短?
I die inside a little every time Duolingo makes me write 點兒 to get an answer correct. Not all Mandarin is Beijing-style. Haha.
So would this mean the same thing: 这条裙子有短?
No, you still need the 點. The 兒 is a colloquial regional northern (Beijing) way of speaking and nearly never included in writing.