I have bought some hair straighteners, but I need to get a step down converter because they are American 110v. I have found a web site which supplies the converter, but do not know which one to get as they range in size from 100watts to 3000watts, depending on what you run off of it.
If all you want it for is the straighteners, you need to find out their wattage. They should have this on them somewhere, or on the box or instructions. Then buy the converter with the wattage nearest above this value.
Or if it's not on the box, try Googling for the make and there'll probably be a product info somewhere. Just remember to buy one above the rating as even a little bit below could overheat it.
At a guess I'd imagine around 500 watts, but that is purely a guess!