I'm interested in this point. I have no idea, because the only mice/rats I do this way are pinkies/fuzzies. I'm not being facetious at all here, but if it's anything similar to killing a chicken, size shouldn't be much of a factor as long as the proper force is exerted and it's done with a confident hand.
Agreed, I pay $9 to refill my canisters, gas 100's of rats a year, and spend less than $40 a year in refills.