You don't need more than one strop... you don't even really need
a strop... I always suggest getting at least one, but it isn't needed.
As far as size, length can help with longer knives but you can adjust your technique and either will work fine. If you have a 3x8 flattening plate already, get the 3x8 strops. If you need a flattening stone anyway, go with the 3x8. If you are starting clean, I like the looks of the 3x11 but the 3x8 will do the exact same thing all day long. You just have more room to draw is all.
I'd go with maybe around a 1 micron spray (I'm not too anal about this... some are lol) and balsa paired with bare leather of some sort. Probably cow or horse butt. Kangaroo I think works best with a fine compound or spray. Cow or horse butt should do fine bare.
Or just get the balsa and spray and finish on newsprint. Any of it will help to polish up and refine the edge.
I don't really see you having to buy something better, but you might eventually want to add to it later. It isn't called "getting deeper into the rabbit hole" for nothing. lol