Dressing in Iran is dependent on religious and legal criteria. Men and women are required to abide by the following dress code:
Men must wear long trousers. Shorts and three-quarter length pants are unacceptable unless on the beach. Short-sleeved shirts and t-shirts are also acceptable for men.
In public, women must wear a headscarf to conceal the head and the neck, a long shirt with a collar coming to the base of the throat and the bottom reaching to just above the knees, long sleeves and long pants. There is no rule for the colour of the outfit.
Travellers can easily wear the loose fitting clothes sold from Indian or hippie-style shops where the tops are loose long sleeved (tunics) and cover your bottom and a loose scarf for your head. You can wear any type of pants underneath as long as they cover your ankles. Women are allowed to wear open toed shoes and sandals as long as their ankles are covered. Typically, wearing a dress over a pair of pants works as well, if it follows all the other rules, too.
Here are some photos for your consideration: