Any size logs will work. You can use branches or saplings if that is what you have available. Small-diameter wood will colonize faster, but will not produce for as many seasons as a larger log. You don't want the logs to be so large or heavy that they are difficult to use. For drilling methods, a 4-6'' diameter with a 3-4' length is ideal. For the stacking method, they can be up to a foot (or more!) in diameter, and 12-18'' high.
Fun fact: Your logs can produce for one year per inch of diameter!