red oak
Minister of Fire
Those rounds will be fine to burn, and what size depends on how much work you want to put into it. You could pick up sticks all day for kindling. Like some others have said I usually take down to about 2" when I'm in the woods but if I'm tired I don't bother. In terms of burning I do have some rounds that are 5-6" as they are good for overnight fires, but I usually split those once if they're not too hard.