What is the cut-off for quantum superposition? Is there an explanation as to how macro objects aren't in superposition? At what size do objects stop being in a state of superposition?
 A: There are many models that attempt to explain this quantum-to-classical transition, but the main idea behind most of them is something called decoherence.  I'm not an expert on the subject (yet!), so I won't try to elaborate further but you can get started here. For a more complete treatment of quantum measurement and decoherence check out Wiseman & Milburn, it's pretty good.
What I can tell you though, is that there is no known "boundary" that divides the quantum and classical worlds, and under highly controlled conditions in labs, larger and larger objects are being put in relatively 'clean' quantum states. In this work a silicon chip roughly the size of a red blood cell, consisting of millions of atoms, was cooled to its vibrational quantum ground state. In a later paper this group indirectly detected the zero-point motion of the oscillator, showing that the object really was in a non-classical state! Of course, this object is still quite small, but it is massive compared to textbook quantum systems like atoms and molecules. 
So to answer your question, so far all the evidence seems to indicate that putting full-out macroscopic objects in a quantum state is possible in principle.
A: A quantum stops being in a superposition of different eigenstates for a certain observable until it is measured. After the measurement it is in a certain eigenstate. 
If an quantum interacts with its environment and gives information about its state to the environment this is already a "measurement".
A certain border for a size of object for which superpositions of states can exist can not be drawn.
