uxcell PVC Water Pipe Clamps Clips, 1-1/4" (40mm) TV Trays Tubing Hose Hanger Support Pex Tubing, 20Pcs

Q: What size will hold 1/2in sch 40 pvc

I bought this product (25mm inner diameter) to hold 3/4in and it did just fine. For 1/2in you will need to buy the smaller. I imagine the (20mm inner diameter). If you're ever unsure you can google many pvc size charts in mm all over the internet.

Q: I don't understand how these clamps are to be fastened to anything using a 5 by 8 mm hole... Are there special fasteners for this?

I simply used a #6 X 3/4 (.75) screw to fasten each one to my fence. They are used to hold two 100' hoses which run around the garden. Been up for over two years now and work (and hold) just fine.