.54 to .75 smoothbores get 2-1/4" to 2-1/2" patches. Patch thickness needs to be whatever allows a "snug" fit so that it will apply a "scrubbing effect", even when wet with water or Ballistol or WD40. Patching that appears too tight and are of a goodly size can be placed off center on the muzzle to make them easier to push down/out.
Bbls that don't seem to want to come "clean" (light grayish look), or have leading that doesn't seem to wanna come off, can benefit from the Lee Shaver bbl cleaning treatment of making up a special patch by unrolling and cutting off a width of 0000 steel wool, place over a patch, saturate with oil, push down steel wool forward, and pull out. The overall thickness of this cleaning patch must yield a more than snug fit that will initially require some serious effort to push down past the muzzle (but will go down easier after that). It helps heaps to use a brass rod. Extract the cleaning patch and throw away, make another one, After using up an entire steel wool pad, yer done. Not only will the bore come clean it will be lapped it at the same time. Lee has done this process to brand new bbls and without even shooting the gun in practice has won matches (well, it helps to have a world class shooter eye and trigger finger, tool!). I've done the Lee bbl cleaning to more than a few old and new guns and he's spot on with the results.