i shoot = parts 40 grains or 3FF with = shot , i think if i recall that comes to 1/2 to 5/8 oz of shot
as to the largest shot size , that would without a patch be .410 round ball or with a patch .395. I used to shoot 40 cal balls all the time in mine .
as to the shot size , i dont think there is any set size you cant use but i found i liked # 6 to #7 1/2 shot , for grouse pheasants and chucker size birds . with phesants i worked up a hot load heavy on the shot side .
what type of .410 do you have , i been looking at building a SXS in flint for grouse hunting