Mine were 8mm on the examples below, which i think are the perfect size.
Note you will have to dremmel down the backs of those holders as they're deeper than the reflector.
You need to secure the holders in the reflectors with glue, but not superglue as it reacts with the plastic. I use 'serious glue' which they sell in Wickes for a few quid.
Lastly, i always filled mine with clear silicone sealant to waterproof them, before sticking the back of the reflector back on top (which i'd previously dremmelled off.)