We shoot greylag, which is smaller than canadagoose (I think), and a Pink-footed goose, which is smaller than the greylag. I tend to use nr 2 or 4 pelletsin 42 gr. or 3 inch shots but some are using 36 gr. loads and wait for a closer range.
I've never known in reality why the 10 ga. was banned, but I think it has something to do with hunters using tens on seals and small whales some hundred years ago. As I said I don't really know, and no lawyer or official has ever been able to tell me actually why. Rifles weren't common here until after WWII so that might be the case.