Personally I wouldn't use cz for either cooling or heating, especially heating, there is just too much variance in the manufacturing and I can afford purer choices.
As for the size for downstems, don't bother! If it's intake for a wp then just let the water, or the geometry of a dry piece, do the work. Size in any cooling stems should be pretty subjective. The larger the balls the less draw resistance, but also less surface area meaning warmer vapour. If you're considering them for a restrictive vape, then go smaller because they will have less impact, if you're considering them for a hot vape, then consider how the vapes resistance is for your preference. If it's too open, choose smaller balls, if you like the openness, choose larger balls but use a longer stem.