Many times it’s the locals that are misrepresented.
There’s no travel blogger who has visited Iran and hasn’t written about how he was surprised to see things where not as he imagined.
As long as they cover up your bum you’re good to go (I’ve seen tourists wearing shorter ones with loose trousers too).
If you don’t have a tunic, you might find a long tank top that can be layered up with a light cardigan.
While there is a criteria for how you dress in public, you’ll find women to be dressed in many different ways.
Some just wear a loose narrow scarf on their heads allowing whips of their hair to frame their face with untied hair also showing from the back, some will be with heavy make-up with their hair styled where it is shown, others might wear it tighter with hardly any hair showing and there are many women who prefer to wear the Chador.
You will see women wearing black Chadors but chances you’ll see someone with a veil is almost zero in Iran.
It’s just not in the culture, no matter how much someone might be conservative.
Check out Ascia, a kuwaiti/American fashion blogger here.
And Leena a Texas based muslim fashion blogger who has a beautiful choice of colour scheme in her outfits.
Iranian women either cover up with a chador (The black material worn on the head that covers the whole body) or a Manto (A trench coat type of clothing that comes in a variation of materials and styles).
Since you’re likely to opt for the Manto buying one at the beginning of your trip could be a good idea.
By almost a 100% chance he’ll write about how Iranians were super nice and one of the most hospitable nations he has met. I get that what goes on in Iran might not come to your liking. A lot goes on here and certainly not all of it is good.