Hello, I am changing the Contact set (points) on my 83 holden Jackaroo and need to know the gap size to set the contacts to.

An old-timer tool for this minor adjustment is a common book of matches: the gaps is approx the thickness of the book and the striker for burnishing the contacts, if necessary.
I would begin at .016 - .020"/.4 - .5 mm and then fine tune be ear... see how it runs with that setting. You can then do a finer adjustment be ear.
I hope this helps.