How does steric hindrance cause sulfur hexafluoride to have almost no reaction chemistry?
As you can see:


 Sulfur (16) is significantly smaller than Se (34). That size difference
 means that there is more room between the fluorine atoms on Se than on 
Sulfur. That is what steric hindrance means. Think of steric hindrance
 as limited access. The smaller the atom the less access there is. 
Oxygen is so small that less than 6 fluorine atoms can crowd around it 
because of steric hindrance... probably more like four. 

So because there is more room between the fluorine atoms on Se there is 
more opportunity for other atoms to attack the central atom Se, but 
sulfur is completely shielded by inert fluorine.