yes, females grow way slower and same age males and females will have very big size difference. as well it is true, males and females can live together, but you can never relax if you have 2 or more frogs in one tank, you need to constantly monitor their behavior. I don't mean go all OCD about it, but just watch them.
i would say if your male is 6-7" and female 4-4.5" - that is ok for sure ( as sure as it can possibly be), 8+" male and 3" female - absolutely not ok.
even for similar size frogs make sure you have a big tank, so frogs will have possibility to get out of each other way, the more crowded it is the more aggression you'll see. actually same sex animals will fight if housed in small in comparison enclosures.