It’s not just about size. Maturity is very important. A 16 week old pullet is not very mature compared to a full grown hen. Mature hens outrank immature pullets in the pecking order. It doesn’t matter if the pullet is a Jersey Giant and the hen is a bantam 1/3 its size. Size does play a part, but maturity and the willingness to fight back due to that maturity plays a much bigger part. The pecking order is about personality more than size and adults have a stronger personality.
What you are seeing is perfectly normal. When two chickens meet that have not settled the pecking order, one normally pecks the other or somehow tries to intimidate it. If one runs away then they have helped work out the pecking order. There might be some chasing and there will probably be a few repeat lessons, but it normally works out very peacefully if one runs away.
If one does not run away, that is considered a challenge to the pecking order rank. That’s where you may see some fights. Even if they fight, it usually does not take long for one to decide that it is better off running away than sticking around and fighting. Usually it does not come to fighting, just some pecking and running away.
That’s why it is important for them to have enough room to run away. If the weaker does not run away, the stronger thinks it is a challenge and will continue pecking and attacking. The weaker may just sit down trapped against a fence or wall and not even fight back or try to get away. This is normally how you wind up with dead chickens.
Don’t let that scare you. Yours are free ranging. They have lots of room. Many of us integrate chickens all the time without serious problems. I normally integrate 30 to 35 chicks every year that I hatched in an incubator and raised in a brooder until they were 8 weeks old. At 8 weeks I turn mine loose to roam with the flock. They normally get pecked when they get too close to the adult hens, but soon learn to stay away from the adults so they don’t get pecked. I have never had one killed due to integration, but I have lots of room. If my available room was less, I’d wait until they were older. At 16 weeks yours should be plenty old enough especially with them free ranging.
I think Egghead gave very good advice. Put them where they have the most space so they can work out the pecking order. You will see some pecking. That is perfectly normal. You may see an older hen chasing a pullet some. That is normal. But watch them for a while. As long as they do not draw blood, leave them alone. That’s how chickens determine the pecking order. They peck.
If that one hen continues to attack like that and just won't quit, try locking her up by herself for a couple of days if you can. That way, they often have to work on getting their own pecking order rank back and don't single out the newcomers.
They will probably go into the coop at night to sleep with the older hens. You probably want to check and see where they are sleeping. Often younger pullets like that will sleep somewhere other than the roosts. An older one may make life so bad on the roosts they sleep somewhere safer. That's perfectly normal, mine do it a lot. But if that safer place is in your nests, you might want to do something about that. you don't want them sleeping in your nests and pooping.
Also if you lock them up at night, I'd get up early a couple of days to let them out early. Since you have that one hen that is not very nice, the pullets might appreciate a chance to get away. What I expect you to see when you go down there is that the older hens are on the coop floor and the pullets are still up on the roosts, trying to stay away from the adults. Or the pullets may be hiding under or behind something, just trying to stay out of the way,
It sounds like you have a good start on a successful integration. Best of luck the rest of the way.