There is nothing below plank length. Just like quantum mechanics quantizes energy into discrete portions that can't be divided, it also (theoretically) quantizes spacetime too. Plank length is so small as to be pretty untestable as far as I know though, something like 10^-31 metres, but I may be wrong on that. You're right that QM works on all levels, it's just a statistical mechanism.
I'll give an analogy (I think I may have given this somewhere else on the site but i'll give it again). Imagine you have 2 dice and throw them. If you throw them enough times you could draw yourself a probability distribution of the average score you got. You would find that it would be a smooth curve peaking at 3.5. You would have the most chance of averaging 3.5, but still very high chances of getting numbers either side. The least likely average would be one and six.
The reasons for this is that with 2 dice, the most likely score to get is 7. There are many more ways of scoring 7 than any other number, eg 6+1, 5+2, 4+3, 3+4, 2+5, 1+6. There is only one way to score 12 and only one way to score 2....therefore you're more likely to average 3.5 than any other number.
Imagine then instead of having 2 dice you have one billion dice, and do exactly the same experiment. You throw all these dice and work out the average. If you drew the probability distribution diagram for this you would still find it peaks at 3.5, but it would no longer be a smooth curve. It would be so steep as to appear to be a straight line at 3.5. Now that is only with 1 billion dice, imagine how many particles there are in a macroscopic situation, countless more than 1 billion. In just one mole of a substrance there is in the order of 10^23 atoms (it may be 10^24, I don't remember lol).
So as you see, on a very small scale quantum mechanics is very unpredictable because you're dealing with only a few particles, but when you go to a macroscopic level it becomes very much more deterministic, the chances of observing something contrary to expectations become so small as to be unimaginable (such as you quantum tunnelling through your chair). One of my courses at university was statistical mechanics, that is where I got the dice analogy from. In this course we derived much of thermodynamics using this analogy and some pretty clever but at the same time simple maths
As nosyned said there are quantum mechanical effects that can be observed on a macroscopic scale. Bose-Einstein condensation is one, quantum entanglement is another (I remember reading somewhere that they've managed to entangle a container of caesium ions for a considerable amount of time). This leads on to teleportation, which is also quantum mechanical.
As to a barrier between micro and macro i'd say its just a continuum. The bigger in scale you get the more macro-like things become etc. Much like in evolution, things just merge together and we're left with the impossible taks of classifying things into one group or another hehe.