RE: What steel shot size?
Your dad was a duck hunter, he was right on the lead #6's, and, the good old days are still here. Hevi shot #5 or #6 are effective, but the pellets are as hard as steel and will wreck your teeth if you hit one chewing. Matrix and Bismuth are both just as effective, and crush when you hit them during dinner.
Spend the extra $$ for these shells. You won't need to shoot many cripples and will spend more time in the blind during prime time rather than chasing wounded birds around the pond.