Are you not intending this gun to be used for hunting of any kind?
What ranges are you wanting to shoot? Distance has a way of limiting the caliber of ball we would recommend.
Length of barrel, even when swamped, is going to depend on your body shape and size, your ability to hold any gun steady, and what makes you comfortable shooting off-hand. There is NO one-size-fits-all.
For target shooting only, I would recommend a .40 caliber, as a Nice compromise, with the .45 being my second choice. I am slightly over 6 feet tall, but found that a barrel of 38-40 inches is muzzle heavy enough to give me good steady "barrel hang". If you are much shorter, you might be served better with a gun that is a few inches shorter in length.
The off-hand shooting I did, and still do involves targets at 50 yards and less. I hate punching paper, so have no interest in doing the kind of daily practice needed to get good enough to shoot 100 yard target matches.
The Best shooting I have done on paper was a 47-2X( 4-10s; 1-7-called dropped shot) shot off-hand at 50 yds on a cold New Years Day, shooting a .45 caliber percussion "Carbine" with only a 25 inch barrel. I used 50 grains of FFFg Goex, behind a .445 ball, and .010" cotton patch. Lubed with spit, as I fired the gun within seconds of loading it.
I have done a lot of accuracy "trick" shots using my .50 cal. flintlock, with its 39" barrel( straight, not swamped)but never have shot a better "Paper" score than I did that day with my short barreled .45. I think the best paper score I have shot off-hand with my .50 was a 42-1X. I bought this rifle for hunting, not target shooting, so I simply have not tested the gun on paper very often.
If you are looking for a gun with a certain length, it would be worth your while to get to Tip Curtiss' place in Tennessee, or see him at Friendship, or at some of the other events he has. He has dozens of guns of all sizes with him in his trailer, and it would help you greatly to know what "feels" right for you before you buy or order a gun made for you with that swamped barrel. Chuck Dixon also has a lot of guns you can "try" in his shop in PA. I am told that The Log Cabin Shop, in Lodi, Ohio also has a large stock of guns, as do other stores across the country.
Best wishes on your search. Part of the satisfaction with your final choice is all the work you do to find the gun that fits you!