what is the deal with the patch behind the ball and the different sizes of the patch.
The patch is not just behind the ball; it surrounds it on all sides as well. It's a patch, as opposed to a wad.
Long rifle muzzleloader round balls are deliberately undersized for the bore - the patch forms the gas seal between the bore and the ball and imparts the spin from the rifling.
Example: .50 caliber long rifles generally use a .490 or .495 round ball; they are loaded with a patch approximately 1 1/4" in diameter so that the patch folds up around the sides of the ball when it's rammed down the barrel.
The patch thickness is one of the variables one must establish to obtain the best accuracy for your rifle. Patch material (it MUST NOT contain synthetic fibers - 100% cotton pillow ticking is best) is generally available in thicknesses from 0.005" to 0.020". The only requirement is that twice the patch thickness plus the ball diameter be larger than the groove diameter of your rifle.
Assuming a groove diameter of 0.500 and a .490 diameter ball, choose a patch material thicker than 0.005": 2x0.005+.490=.500. A typical load would be a 0.015" pillow ticking patch and a 0.490 ball, for a total diameter of 0.520. That may seem too tight, but the idea is to compress the patch into the groove so that it seals the ball and imparts the spin from the rifling.
With at least two different ball sizes (.490 & .495, for example), and several different patch thicknesses (0.005, 0.010, 0.015, 0.018, 0.020), plus the wide variation in powder loading available you can see the possibilities.
Each rifle (that's each
individual rifle, not each rifle design) has it's own 'sweet spot' combination of variables that will provide the best accuracy; some have more than one. While we can all tell you what works best in our guns, nobody can predict what your gun will like; it may or may not be the same as ours. Your job is to try out the various combinations to see what yours likes.
By the way, don't expect a brand new rifle barrel to be ready to test for it's 'sweet spot'. They take some break in time. I usually expect to shoot up to 100 rounds before the barrel is ready to test. Some folks lap the barrels before they shoot to achieve the same state. Trying to group a new barrel can be very frustrating, so have some patience; put a hundred rounds down range just having fun before you get serious about accuracy.