what size foil squares to wrap chocolate truffles?
Some basic math, assuming spherical candy and the type of foil that fits snugly w.o. twisting the ends (like thin aluminum foil):


3" foil -> covers 0.95" candy w.o. overlap -> aim for scant 3/4" balls
4" foil -> covers 1.27" candy w.o. overlap -> aim for 1" balls
6" foil -> covers 1.91" candy w.o. overlap -> aim for 1.5" balls


I figured 3.14 for Pi would be precise enough, and the formula is:


  circumference = diameter * Pi


For cubic candy: 


  circumference = 4 * side


For other blocks (or close enough, e.g. egg-shaped):


  circumference 1 = 2* (height + lenght) 
  circumference 2 = 2* (height + width) -> use the bigger value for square foil


Of course, you'll want to have a foil slightly larger that the circumference, as you need some overlap - just add 1/4" - 1/2", depending on how precise you wrap. 


If you are using the plasticy type of stiff foil that is to be twisted, add at least 1-2" per twist, up to 3" for a more "flashy" effect.