Normally, on the bottom of the barrel, back under the forestock and close to the breechplugs, you will find stampings that indicate bore diameters, and choke. These often are in millimeters. Using Calipers to measure the inside diameter of each muzzle is the fastest way, unless you have a simple bore gauge, a triangular shaped piece of sheet metal, with markings on its sides to indicate what size gauge and then what size choke you have in a given gun, from .410 to 10 gauge.
A Cylinder bore 12 gauge is nominally .729" in diameter. Expect a Modified choke to be about .015", and full choke to be closer to .030". Improved Cylinder will be about .010" of constriction.
As to loads, the advice is correct. This like carrying around a reloading press and components along with the gun. Instead of reloading shotgun hulls, everything goes down the barrel, one round at a time.
you can load the gun as light as you want, to duplicate loads from the 28 gauge, or stoke it up to 10 gauge levels, fairly safely using FFg Black Powder. I will not speak to using other powders in that gun.
My 12 gauge likes 2 3/4 drams of FFg powder( 76 grains) and 1 1/8- 1 1/4 oz. of shot. I have been loading the traditional 1/8" thick OP wad, then a 1`/2" thick cushion wad, the shot, then one OS card( about .010" thick). The cushion wad is too heavy if soaked in moose juice, so I began splitting a cushion wad in half and putting one each in the two barrels, after pressing out the liquid. That helped get rid of the following wads, and the donut hole patterns.
Then, Iron Jim Rackham, from New York, wrote a post here where he only uses OS cards for all his loading, using 4 os cards in stead of the OP wad and cusion wad, and then 2 os cards on top of the shot. He recommended using an awl to punch an off-center hole in the cards, and then align the cards as you load them so that the holes DON'T line up with the one next to them. He followed that advice with some pictures of his before and after patterns, show nice, even, hole free patterns using this method. He puts a round ball of waxy lube between his 3rd and 4th cards behind the shot.
I tried his method, but didn't find a good consistent wax/oil mix for my lube. So, instead, I used a greased cleaning patch to lube the barrel as I seated the final 2 OS cards on the shot. It was speedy, let me carry a minimum of "stuff", and I got very nice patterns. I was a little surprised to see my pellet count increase, and leading in the barrel cease when I used this new method.
I am used to having a second range rod set up with a bore brush, and lead solvent on hand to use to get the lead out of the bore of the shotgun, but they are not needed if I lube the bore IN FRONT OF the shot.
I do run a cleaning patch down after each shot, slightly dampened on my tongue, and that seems to be enough to clean the crud out of the back of the barrel so that the gun remains "one shot dirty".
The nicest part of using only OS cards is that they are thin enough, but strong enough, that you can bend them to get them past the choke in your barrel, then turn them, while not destroying the edges.
I am using 2 3/4 dram FFg powder and 1 1/4 oz. of #5 shot for hunting pheasants. I use the same powder charge and 1 1/8" of of #8 shot for dove, quail, and partridge. The #5 shot works well for rabbits, and for squirrels, altho any shot from #8 to #4 will work for both of them. I find that #5 shot are heavy enough to completely penetrate a pheasant, but if a pellet doesn't, its large enough to locate easily when cleaning the birds, or preparing them for cooking. The smaller pellets can be a real test of your patience, and sense of touch some times. I bought some nickel plated #5 shot to use, and I expect them to penetrate even better. Given a choice( ie. knowing that I am only going to shoot rabbits, or only squirrels), I will use them for squirrels, and rabbits, too.
I try to keep my MV down below 1100 fps. That seems to help the patterns stay round and tight. Air is not only dragging on the pellets as they fly through the air, but its also trying to push them away from each other. The faster the MV, the faster the pellets spread out. With choke, that process is delayed, which is why choke is desireable, but it can't stop the process. It delays it just long enough at the short ranges we shoot shotguns to allow tighter patterns at 30-40 yds. With my cylinder bore gun, I consider 25 yards to be a " long " shot. But, I have killed pheasants out to 33 yds with the gun.