Is there a "Size" Cutoff to Quantum Behaviour? We all know that subatomic particles exhibit quantum behavior. I was wondering if there's a cutoff in size where we stop exhibiting such behavior.
From what I have read, it seems to me that we still see quantum effects up to the nanometer level.
 A: There is no known cutoff in "size" where systems stop exhibiting "quantum behavior".  I put those words in quotes, because "size" can mean different things to different behavior, and you weren't explicit about what you mean by "quantum behavior".
Folks have seen double-slit interference with molecules made up of ~60 atoms (I think recent experiments may have increased this number).  If you were to try to do the same thing with a baseball, as far as anyone knows there's no fundamental reason why it wouldn't work, but the experiment is far beyond our technical abilities (it would take a length of time greater than the age of the universe to do the experiment, nobody knows how to isolate the baseball from the environment for that length of time, etc. etc.).
Superconducting rings have been built with diameters of a few centimeters (probably bigger) and still show flux quantization, which is a quantum-mechanical effect due to the electron wavefunction.  Those rings are definitely bigger than a few nanometers, and I'd consider it "quantum behavior".
Folks have shown quantum-mechanical entanglement between two light beams that were separated by kilometers (in EPR tests).  A kilometer sounds like a big "size" to me, but maybe that's not what you meant?  
Take your pick as to what's the biggest "size" here, but as far as anyone knows, there's no size cutoff to quantum mechanics.  However, depending on the kind of experiment (2-slit interferometer, etc.) it definitely becomes more and more technically difficult to perform the experiment as the system is scaled up.
A: There are definitely circumstances where we see quantum behaviors at rather large scales like, for example, in superconductors. In BCS theory cooper pairs are described by a macroscopic wave-function which, to my knowledge is valid for the bulk superconductor no matter how large it is. 
A: The classic experiment demonstrating quantum effects, the 2 slit experiment, has been preformed with subsequently larger and larger particles as our technology available to do it advanced.  Originally, it was performed with electrons, which are just as much matter as any other matter, but are extremely small.  The largest particle it has been demonstrated with are Buckminsterfullerene, which contain 60 Carbon atoms.  For size comparison:
$$m_e = 5.485 x 10^{-4}u$$
$$m_{buckyball} = 720.642 u$$
There is a good reason that the experiment gets more difficult with increasing mass, and to be sure, the buckball experiment was quite an accomplishment.  To the basics of quantum mechanics:
$$\Delta x\, \Delta p \ge \frac{\hbar}{2}$$
Alternatively, the de Broglie wavelength is:
$$\lambda = \frac{h}{p} = \frac{h}{m v}$$
I believe that in order to obtain the same wavelength with the same mass you have to decrease the velocity.  The reason this could be problematic for such experiments is that it is hard to successfully create the conditions needed with a large and slower moving particle, such as needing a better vacuum.
