if you are ordering from a nursery or such, which i would be at that quantity, you would order by cubic meter (m3)
so, width x length x depth.
assuming your depth would be 50mm-100mm, you would need 0.5m3 - 1m3 respectively.
you can generally get 0.5m3 lots and that is generally what safely fits in a trailer, so you could try 0.5m3 at first and return for a second load if you are short.
Welcome to the Bunnings Workshop community @malkyfaith. It's terrific to have you join us and many thanks for your question.
It's great to see you receive a comprehensive reply from @HandyAndy and I trust that answers your question.
I'm sure our members would be interested in hearing more about your project and we would encourage you to let us know if you need any assistance or if you would like to share it with the community. You'll find plenty of inspiration for all your house and garden projects here with our amazing members contributing their own garden makeovers all the time.
We look forward to seeing the results after you cover the area with pebbles.