Modesty in clothing is not only the length of your skirt, but includes being appropriately dressed for the occasion.
For me, some no-nos would be, revealing and sexually suggestive clothing, which includes short shorts and skirts, tight clothing, and shirts that do not cover the stomach.
I believe women should wear clothing that covers the shoulder and avoid clothing that is low-cut in the front or the back or revealing in any other manner.
Skirts should cover the knee when seated. Jeans are fine if they are loose.