The holes are 3mm (M3 screws). Depending on the material this will need to be scaled to accommodate shrinkage.I'm using 12mm M3 screws (fyi, that's not including the head).
To make the blades (I bought these third party ones ― https://www.amazon.com/gp/product/B01MV0MPJH/ref=oh_aui_detailpage_o09_s00?ie=UTF8&psc=1 for around $15) fit in the adapter, you'll need to remove them from the quick release. I just cut the section of the center of the quick release, then the blades came off the pin. The holes in the props are too small to fit the screws through, but the plastic is relatively soft and it only takes a few seconds to use a screw to enlarge the hole so that it does.
Once finished, run screws through the top, then the blades, then the bottom. You can attach a nut if you feel you need to, but I have had no issues whatsoever―everything is tight enough that it Isn't a problem, plus the nylon nut that holds the blades to the motor really all you need in this case.
Side note for those interested, The span of the blades is increased by less than 2mm.