There's quite a bit of information on female clothing in Iran on this website. What about shoes? Should the toes be covered or the whole foot? Or are sandals acceptable nowadays?
Female tourists in Iran has to cover their hair appropriately. They should also wear loose fitting garments which cover arms and legs up to at least the knee area over pants or skirts with a scarf in summer, and a full-length woolen coat and scarf in winter (calf-length is acceptable if worn over pants).
Dress attire for men is similar to that in Europe. The strict dress code for men in Iran prohibits short trousers and short sleeved shirts and t-shirts. Although you would not see Iranian men dressed in shorts, you will almost certainly see men wearing short sleeved shirts and t-shirts.
Sandals and flip flops without socks is acceptable for both genders and makes no problem.