I have several friends your height and shorter, and they can easily handle a 28 or 30 inch barrel. You are unnecessarily limiting the amount of powder that will burn in your gun choosing such a short barrel. At 21", you burn 72 grains. Compare that to just 28", which can burn 97 grains!
It will be the light weight, and the balance of the gun that will determine how much recoil forces you will have delivered. Then, the width and style of the butt plate will also determine how much of the recoil you FEEL.
In my 20 ga. fowler( smoothbore- not a rifle) a 75 grain charge of FFg powder gives me a health SHOVE, but not a sharp jolt to my shoulder. I have a shotgun style buttplate, as opposed to the half moon, " Hawken-stye", buttplate so commonly found on many rifles.
The buttplate on my gun is much wider- more than 2 1/2" at the widest point-- and that spreads out the felt recoil. I have not tried shooting 100 grains , or even close to the 111 grains my 30 inch barrel will burn in the gun.
With that 3/4 oz. RB, those heavy powder charges are simply not needed to kill any deer, at the short ranges where I am going to hunt using open sights on the gun. The heavy ball is going to pass completely through the deer on all but a chest-front shot.
I also think you are going to find loading a short, 21 inch barrel a royal PITA in the field, because even you will be leaning over a LOT doing so. I recommend that you choose a longer barrel length for this gun. You will appreciate it.
If you have never had formal lessons on shooting stances, and learned the difference between a rifle stance, and a shotgun stance, you need to find someone to do that kind of coaching some afternoon for you. It won't take long. But, I found I appreciated the longer barrels when I learned how to properly stand with both kinds of guns.
I have a 22 inch DB shotgun I still shoot, and my 39 inch barrel on my long rifle, so I do have some practical experience shooting guns with different barrel lengths.
