Go to Track of the Wolf Site, and order their 28 gauge wads.
They are nominally .550, but the outside diameter will be .010" larger than that diameter.
Starting loads begin at 1 3/4 drams( 48 grains) and 5/8 oz. shot. Most go a bit more than that and use 2 drams( 55 grs.) of powder( FFg), and either 5/8 or 3/4 oz. of shot. The longer barrel is going to burn up a lot of powder, so I recommend starting with the 2 dram/ 3/4 oz. load and seeing how it patterns at 25 yards. It the pattern is ragged, then back the powder charge down first, to see if it tightens up.
I also recommend that you lube the bore with a greased cleaning patch after seating the OS card on the shot.
For Round ball loads, Start with 55 grains of FFg powder, a .530 RB, and a .015 lubed patch. Again, grease that long bore with lube after seating your ball. ( You can put the greased cleaning patch in front of the jag and both seat the ball and grease the barrel efficiently with ONE stroke.) Some people are shooting up to 70 grains from their 28 grain barrels, and I am sure your new gun will handles such a load. Whether you will get better groups out to 50 yards with a PRB and that much powder is for you to determine with the individual gun. ( And, I am sure that someone will recommend a lot heavier load than these just as soon as I post this, calling me all kinds of names. To each his own. I found out a long time ago with BP shotguns that more powder is rarely the answer to better patterns or accuracy with RB loads. You might check out the load information you can find at Bob Spenser's Website, " BlackPowderNotebook" ,
http://members.aye.net/~bspen/index.html
And Read the Article by V.M. Starr, listed at the bottom of the second list of articles in his opening page. Starr gives loads for a lot of different gauges, and good advice for all shooters, based on years of experience. )