Knee length or slightly below is fine. Speaking from a purely personal view, i love the colourful summer dresses.
Why are they better than skirts? Because they are always fuller in shape and allow easier lifting to view retro undies. Remembering the Can-Can dancers from the French Follies, we don't need to go that far, but the concept is good and provides something to aim for.
![grommit](images/smilies/grommit.gif)