T-Bone,
I have only seen two swans shot, and they were illegal. Some kids down on the Boise shot them while we were across the river. They killed one, and wounded the other. I called the game warden, and he came out. We had to chase this big ol' Swan all across a hay field, and into a creek. I finally caught the Swan, after getting beat to death by the wings.
They are a dang big bird, and it would have defeated the purpose if I had hit back, so I just kept getting beat, until it was subdued.
But, as for the shot size, for Geese, I used to be a big fan of T's. Then I started using BBB's, then BB and started thinking more were dying. Then I got some 1's, and had a couple of good days. And then 2's.... GirlGunner used 20ga 3" #2s for her one shot/one goose.
Here's my current thinking, and I may be all wrong. There are two ways to put a goose (or swan) down. A "kill shot" to the head/neck, or a broken wing. Shots to the body will not always knock them down, as I have seen plenty of feathers fall out of birds that never missed a beat. That being said, I do not believe it matters if it is a headshot, if it is a T or a #2. They are both big enough to kill. Look at how we kill Turkeys.
For a Wingshots, I think the chances of breaking a bone are greatly increased with the added number of pellets sent toward the goose (swan). And I believe #2's still have adequate energy to break a wing bone. I still prefer BB for Geese, but I will not hesitate to mooch a #1 or #2 from somebody else, and kill geese.