I think people expend a lot of bandwidth on this topic. There are almost always small twigs around that can be used along with various sized twigs and branches becoming progressively larger.
I would suggest that the real answer for a lot of people simply comes down to the choice of what they WANT to do. Sometimes I want to process a bigger log, so I carry larger tools. Most of the time (when I really NEED to make a fire), I could use practically any size knife to make the shavings I need (even if the wood is wet...just keep whittling to the dry stuff).
Perhaps there are areas where the only thing that exists are 5+ inch rounds that are soaked to within a few inches of the core...but around here trees grow and die with smaller branches that can be broken by hand and careful foraging will usually turn up dry material even on wet days. YMMV
When I'm out with friends I usually like to drag dead trees out of the bush and make bonfires. The only tool I use is a Bic.