Got a helpful pm containing some good info that I thought would be good to share. After a little research and considering my particular application I chose to purchase some 8mm prusiks. Also purchased a petzl tibloc as it looks like a useful tool. Thanks to all the contributions of knowledge. Hope this may come in handy for others when assembling a z drag kit.
Everyone seems to be saying 1/2 the size of your main rope. Rather than 5 or 6mm, I use 8mm for prussik strength.
Here's some of the research that I used to decide 8mm over 5mm.
Like was found in the testing/experiment, I have experienced prussiks NOT slipping on a dry main line/rope.
I have only anecdotal evidence/recollection about the prussiks slipping on wet rope. Certainly not consistent or predictable.
Having had smaller prussiks fail under load, I decided that prussik strength was important and have increased the size/diameter of the prussiks I(we) carry in our wrap kits that use an 11mm static main line. YMMV.
You might want to investigate the French Prusik and the Bachmann Hitch. There is some evidence that suggests that those knots/hitches place less stress on their own cord and therefore may break at a higher load instead of the lower breakage point of a traditional Prussik Hitch.
I(we) also have a big river wrap kit (bigger, wider rivers, longer 400' main line) that uses a 13mm static main line and 9mm prussiks. I have also used ascenders and Rescuescenders in place of prussiks in wrap situations, but now try to avoid those in high load scenarios as main line sheath damage seems to always happen. Prussiks and especially double prussiks seem to not damage the main line as often.
Hope all that helps,
Good luck in your search,