I think that the size/weight that any motor can handle is a lot dependent upon how well centered the weight of the dish/lnbf is around the motor's axis of rotation. This is one more reason that the bent shaft on the motor shaft is important, because it allows the dish to sit lower, at a point where the extension of the motor shaft rotation axis would be.
If the center of gravity is centered on the rotation axis, it really doesn't take much force from the motor to turn a heavy dish, but if the dish is extended high above the axis, then it could take considerable force even from a light dish. I think that this could be an issue with respect to the U-bolt and pole mount brackets that
Sadoun sells. Ie these seem to hold the dish out at different distances from the shaft, so each of these might be better suited for different dishes, depending on how their weight is distributed.
On my big 10' dish, which sticks WAY out above the rotation axis, in the winter, I have a hard time getting it to come back to south after I've turned it to the west, so I have a big log attached as a counter weight. It's not really pulling in the right direction, but it helps just enough to allow me to move the dish. So if the
SG2100 isn't able to move a particular dish, one thing you can do, is add counter-weight to center the weight around the rotation axis.