Pot size will be dependent upon which amendments are allowed, and how often your mix can be amended.
If it is just make soil and water only for comp, you will likely need 7 gallons minimum to make it all the way through without amending, and even then…
If it is single amendments additions only, such as meals or castings, you will need to make sure you have significantly more aeration to your soil to start, as those amendments will “weigh” your mix down, and cause drainage issues the further along in your grow that you get.
Will organic fertilizers be allowed in the initial making of soil, or will it be strictly mineral and media inputs?
I like to cook my soil for at least 30 days, but prefer much longer.
Lastly, there will be a pretty significant barrier to entry for those not currently running living soil, and have to make.
Not trying to be a Debbie Downer. Just things to consider. Depending on timing, I would most likely jump in this “competition/challenge” as well.