In order to maximize the strength and durability of a piece, it's a good idea to use the thickest version of your chosen stringing material that you can easily get through your beads.

Of course, you need some wiggle room in there, so keep in mind that a 1.5 mm hole cannot accommodate a 1.5 mm cord. There is almost always some variation in hole size among similarly sized beads, so you don't want to plan for too tight a fit because you might find that a fair number of your beads are just a hair too small. Lastly, if using a needle, the hole will have to be at least twice as large as the stringing material.