When is wood not worth the effort?

4" sounds right unless it has a lot of branches and crooks. I think of it as the size of my forearm.

Well I keep branches down to about 1 inch, great for starting fires, just the good stuff oak, mulberry,ash, the silver maple I keep at about 2 or 3 inch.

I keep everything down to about two inches. I'll even go smaller if it's a tree in somebody's yard, where I have to clean the brush up anyway. All btu's.

All depends how wood hungry you are. Last year I saved almost a whole tree. The small stuff I used for kindling. The sticks I used during the late fall and early spring when I
didn't need big fires.

If I grab a branch end and if my thumb touches my fingers when holding it, its starting to get a little small and small starts getting labor intensive.
My branches mostly get dragged /scatterred in the woods (sometimes I'll come back in 6 months or so and step on them to increase ground contact and rot), there's less walking involved keeping the branches a bit larger.
I can get greedy with the oak, though and want it all. (smaller branches fit in a cart better, too)

The small stuff can be a time killer, though.
Use a stop watch some time and make some piles.
The small stuff won't seem worth it .

Usually it's 3-4 inches for me, I judge with my middle finger. Anything less then that is usually too much hassle for me.

With 200 acres and somewhere to dump the brush, I'd say you're right on with the 4" estimate. Someone on a smaller lot like myself cuts smaller and smaller every year as the brush piles gett bigger. I'm already digging out stuff I've discarded year's before. Nice and dry!

It all burns and produces heat... I tend to use as much of the tree as feasable.... 1" or so depending on where the off shoots are on the branch

Depends . . .

Normally I'll stop bucking at 2-3 inches . . . which is a lot smaller than what my buddy tends to take . . . my own feeling is that even with access to all kinds of "free" wood on the famiily land I hate to waste the wood and figure if I'm taking the tree I'm going to use as much of it as I can . . . with the rest of the smaller stuff eventually breaking down into the soil . . . after providing some habitat for the bunny rabbits and other forest critters.

However . . . if I'm taking down a tree around my house I tend to go even a bit smaller in size . . . I figure there is less hauling and moving . . . and whatever I don't cut up will go on the burn pile and I would rather get some useful BTUs in a burn in my woodstove rather than a bunch of BTUs just going up in smoke for a brush pile burn.

The other time I go smaller is when I find some premium wood that I don't often get or find . . . for example I took down an oak or two a year ago . . . and on the family land we have very few oaks . . . I tended to take smaller wood than I normally do since this wood was a "treat." I tend to do the same with apple . . . while it is more common . . . it is a nice wood to burn or give away for smoking.

On the other hand sometimes I am wasteful and don't cut so small . . . I tend to do this when the tops are just too "brushy", the tops are stuck in a swampy area or I'm just plain tuckered out after a long day of cutting wood.

I guess its weird for me, I have trouble throwing away small pieces of ash or mulberry but have no problem throwing away boxelder and cottonwood trees

4 inches sounds OK for a rule of thumb...I'll go lower myself for some smaller starter pieces though.

I have access to large amounts of wood so I will often find myself leaving big narly trunks and odd shaped pieces unless it is oak. When I have some help it is different but when splitting by myself nice rounds are preferred. The small stuff down to about 3" is nice for shoulder days and kindling. Doesn't fill the truck very fast though.

Oak, Walnut, Elm, and Cherry are the main pickings in my area. Ton of Boxelder but I have yet to cut or burn any of that stuff. It would be easy pickings though.