Generally, women have to wear clothing that conceals their body parts. In Iran you can see most women wearing “manteau” that might be in different colors and even tight or short.
Your other choice can be “tunics” and pants. If you would prefer to wear a skirt, it should be at least mid-calf or longer, with nontransparent, darkish hosiery underneath.
Ankle length skirts are unusual but acceptable. There is no problem with wearing skinny jeans and high boots but avoid Carpi pants, They are too short and not acceptable. Open toed sandals with nail polish are fine.
During late autumn, winter, and early spring (October to March), these requirements are easier to follow, as women visitors can wear a raincoat over pants.
In the warm and hot months (April until late September), my advice would be to choose a linen jacket or even an oversize but ton-down blouse to get to Iran and then have your first shopping experience hunting for a cool overcoat once you get here.
Despite the common misunderstanding, “chador” is not compulsory except when visiting some mosques and holy shrines, where you can borrow one at the gate or even, if fancy takes you, buy a ready-made one from a nearby bazaar.