I have certainly used my grapple (also the 4ft Millonzi) to remove saplings and small stumps with my CK20. The size you can manage are likely related more to FEL lift and curl capacity than the grapple itself. After a year of use, here are my techniques and observations:
1) Saplings. I have found it best to first push them over and thereby exposing the roots. I then put one of the two middle bottom grapple teeth just under the exposed root and drive forward. That usually just pops them out after which I simply close the upper jaw and drive to the brush pile. I have used this technique even with trees up to 4 inches or so so long as I can push it partly over and the soil is loose enough to permit me to get at least partially under the root complex. I have been known to consider it a challenge to take on trees that I really should have just cut down and dug out with the BH and so it sometimes takes pushing at the tree from different angles to loosen it up and I often find my grapple "climbing" the tree with front wheels a couple of feet off the ground in an effort to push the tree over. Likewise, when the root system is a bit tough to get out with a single pass, I will use the grapple to undermine and rip up the roots to the side of the tree before coming back to the main root ball. I also will use tractor momentum by backing up a few feet and ramming the grapple into the exposed roots. Curling sometimes works too especially if there is a long tap root that can be more easily lifted out than broken.
2) Stumps: The same basic technique works for stumps except you don't have the advantage of leverage on the trunk to expose the roots so you need to dig with the grapple a bit more. I find with stumps up to 4-5 inches (soft wood, soft soil) I can use the lower grapple jaw to cut into the ground about 6-8 inches all around the stump before trying to push on the stump to tip it over. Pushing usually reveals where the main anchoring roots are and I then attack them specifically with the lower grapple. Another technique which works well if the stump is fairly low to the ground is to lower the open grapple over the stump and then close the grapple so the upper jaw teeth cut through roots.
In general there is an overlap in size of trees that can be taken down with the grapple vs chainsaw and BH. Bigger ones clearly need the cut and dig approach but depending on the lift/curl capacity of your FEL and the weight of your tractor, you can knock down and scoop out some pretty big trees/stumps. I have learned not to cut trees until I have attempted to push them over and get them out with the grapple as the overall efficiency is much higher if you don't need to use the chainsaw/BH. I then just leave the ones that I cannot get out after 5 or 10 minutes and come back another day with the BH for those.
I'll post some photos later.