There's two things at play here. Stress on the hens. And stress on the humans.
The hens:
A single rooster, over a year old, will generally go over well with the hens. The teenager roosters in particular can be rough and definitely can stress out the hens, especially if there's multiple boys. I do not recommend having younger roosters, as this can create a lot of tension in the flock. A single mature rooster will generally fit in well and the hens will usually respond favorably to him.
I personally prefer hens with a rooster that's their same size. This usually means not the same breed, as roosters of a breed are usually bigger than hens of the same breed. If the rooster is much smaller than the hen, as others have said, he will end up pulling out the feathers more because he's having trouble reaching. I have standard size hens with a very large bantam cochin rooster. Though he's technically a bantam, he's maybe only a pound smaller than the hens. He can reach fine, but makes me more comfortable since he is smaller (he has always been polite to humans, but if he ever wasn't I like that he is smaller and easier to handle).
The owner:
Chicken mating can be triggering for people, since it's not always consensual. I have been sexually assaulted before, and it was uncomfortable at first having a rooster.
Hens don't respond to non-consensual the way we do. They don't develop the level of emotional trauma that we can. They can develop a dislike for a particular rooster who isn't polite, and will chase him off and scold him. That's not to say the hens aren't affected, but they also don't approach mating the way humans do.
Roosters will absolutely hold down a hen. They will also be sweet and take care of their hens, and the hens will also invite him over for mating sometimes. It's not perfect, but the hens aren't helpless, and they do work out their own agreement with the rooster about how things will go. Many species in the animal kingdom sex isn't so nice.
It helped me get more comfortable with having a rooster when I spent awhile seeing flock dynamics in action.
I am very selective about the roosters I allow to stay in the flock. Roosters can have very different personalities, and if they don't settle down and play nice with the hens then they go in the freezer. If you choose to pursue getting a rooster, I suggest you also be ready to try a few different ones so that you can find one that you like.